Map user-specified system libraries to correct variants

Normally users don't specify system libraries such as
libc or compiler-rt on the command line.  However, when
they do it makes sense map them to correct variant.

For example, linking with `-pthread` + `-lc` will now
end up including `libc-mt.a` rather than `libc.a`.

Fixes: #14341

Author: sbc100

emcc.py

@@ -203,15 +203,12 @@ def __init__(self, args):
     self.has_dash_c = False
     self.has_dash_E = False
     self.has_dash_S = False
-    self.libs = []
     self.link_flags = []
     self.lib_dirs = []
     self.forced_stdlibs = []
 
 
 def add_link_flag(state, i, f):
-  if f.startswith('-l'):
-    state.libs.append((i, f[2:]))
   if f.startswith('-L'):
     state.lib_dirs.append(f[2:])
 
@@ -487,6 +484,21 @@ def is_supported(f):
   return results
 
 
+def map_library_names(link_flags):
+  system_libs_map = system_libs.Library.get_usable_variations()
+
+  def map_flag(pair):
+    flag = pair[1]
+    if flag.startswith('-l'):
+      name = 'lib' + strip_prefix(flag, '-l')
+      if name in system_libs_map:
+        lib = system_libs_map[name]
+        return (pair[0], '-l' + strip_prefix(lib.get_base_name(), 'lib'))
+    return pair
+
+  return [map_flag(f) for f in link_flags]
+
+
 def fix_windows_newlines(text):
   # Avoid duplicating \r\n to \r\r\n when writing out text.
   if WINDOWS:
@@ -1054,7 +1066,7 @@ def run(args):
   if state.mode == Mode.POST_LINK_ONLY:
     settings.limit_settings(None)
     target, wasm_target = phase_linker_setup(options, state, newargs, settings_map)
-    process_libraries(state.libs, state.lib_dirs, [])
+    process_libraries(state.link_flags, state.lib_dirs, [])
     if len(input_files) != 1:
       exit_with_error('--post-link requires a single input file')
     phase_post_link(options, state, input_files[0][1], wasm_target, target)
@@ -1115,11 +1127,10 @@ def run(args):
 def phase_calculate_linker_inputs(options, state, linker_inputs):
   using_lld = not (options.oformat == OFormat.OBJECT and settings.LTO)
   state.link_flags = filter_link_flags(state.link_flags, using_lld)
+  state.link_flags = map_library_names(state.link_flags)
 
   # Decide what we will link
-  consumed = process_libraries(state.libs, state.lib_dirs, linker_inputs)
-  # Filter out libraries that are actually JS libs
-  state.link_flags = [l for l in state.link_flags if l[0] not in consumed]
+  state.link_flags = process_libraries(state.link_flags, state.lib_dirs, linker_inputs)
 
   # If we are linking to an intermediate object then ignore other
   # "fake" dynamic libraries, since otherwise we will end up with
@@ -1245,7 +1256,7 @@ def phase_setup(options, state, newargs, settings_map):
         # library and attempt to find a library by the same name in our own library path.
         # TODO(sbc): Do we really need this feature?  See test_other.py:test_local_link
         libname = strip_prefix(unsuffixed_basename(arg), 'lib')
-        state.libs.append((i, libname))
+        add_link_flag(state, i, '-l' + libname)
       else:
         input_files.append((i, arg))
     elif arg.startswith('-L'):
@@ -1987,7 +1998,7 @@ def check_memory_setting(setting):
   # When not declaring wasm module exports in outer scope one by one, disable minifying
   # wasm module export names so that the names can be passed directly to the outer scope.
   # Also, if using library_exports.js API, disable minification so that the feature can work.
-  if not settings.DECLARE_ASM_MODULE_EXPORTS or 'exports.js' in [x for _, x in state.libs]:
+  if not settings.DECLARE_ASM_MODULE_EXPORTS or '-lexports.js' in [x for _, x in state.link_flags]:
     settings.MINIFY_ASMJS_EXPORT_NAMES = 0
 
   # Enable minification of wasm imports and exports when appropriate, if we
@@ -3507,13 +3518,17 @@ def find_library(lib, lib_dirs):
   return None
 
 
-def process_libraries(libs, lib_dirs, linker_inputs):
+def process_libraries(link_flags, lib_dirs, linker_inputs):
+  new_flags = []
   libraries = []
-  consumed = []
   suffixes = STATICLIB_ENDINGS + DYNAMICLIB_ENDINGS
 
   # Find library files
-  for i, lib in libs:
+  for i, flag in link_flags:
+    if not flag.startswith('-l'):
+      new_flags.append((i, flag))
+      continue
+    lib = strip_prefix(flag, '-l')
     logger.debug('looking for library "%s"', lib)
 
     path = None
@@ -3525,22 +3540,20 @@ def process_libraries(libs, lib_dirs, linker_inputs):
 
     if path:
       linker_inputs.append((i, path))
-      consumed.append(i)
-    else:
-      jslibs = building.map_to_js_libs(lib)
-      if jslibs is not None:
-        libraries += [(i, jslib) for jslib in jslibs]
-        consumed.append(i)
-      elif building.map_and_apply_to_settings(lib):
-        consumed.append(i)
+      continue
+    jslibs = building.map_to_js_libs(lib)
+    if jslibs is not None:
+      libraries += [(i, jslib) for jslib in jslibs]
+    elif not building.map_and_apply_to_settings(lib):
+      new_flags.append((i, flag))
 
   settings.SYSTEM_JS_LIBRARIES += libraries
 
   # At this point processing SYSTEM_JS_LIBRARIES is finished, no more items will be added to it.
   # Sort the input list from (order, lib_name) pairs to a flat array in the right order.
   settings.SYSTEM_JS_LIBRARIES.sort(key=lambda lib: lib[0])
   settings.SYSTEM_JS_LIBRARIES = [lib[1] for lib in settings.SYSTEM_JS_LIBRARIES]
-  return consumed
+  return new_flags
 
 
 class ScriptSource: