[lld][WebAssembly] Add RUNTIME_PATH support to wasm-ld

This finishes adding RPATH support for WebAssembly.

See my previous PR which added RPATH support to yaml2obj and obj2yaml:
#126080
See corresponding update to the WebAssembly/tool-conventions repo on dynamic
linking: WebAssembly/tool-conventions#246

Author: hoodmane

lld/test/wasm/rpath.s

@@ -0,0 +1,14 @@
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+# RUN: wasm-ld -shared -o %t1.wasm %t.o -rpath /a/b/c -rpath /x/y/z
+# RUN: obj2yaml %t1.wasm | FileCheck %s
+
+# CHECK:  - Type:            CUSTOM
+# CHECK-NEXT:    Name:            dylink.0
+# CHECK-NEXT:    MemorySize:      0
+# CHECK-NEXT:    MemoryAlignment: 0
+# CHECK-NEXT:    TableSize:       0
+# CHECK-NEXT:    TableAlignment:  0
+# CHECK-NEXT:    Needed:          []
+# CHECK-NEXT:    RuntimePath:
+# CHECK-NEXT:      - '/a/b/c'
+# CHECK-NEXT:      - '/x/y/z'

lld/wasm/Config.h

@@ -70,6 +70,7 @@ struct Config {
   bool pie;
   bool printGcSections;
   bool relocatable;
+  llvm::SmallVector<llvm::StringRef, 0> rpath;
   bool saveTemps;
   bool shared;
   bool shlibSigCheck;

lld/wasm/Driver.cpp

@@ -583,6 +583,7 @@ static void readConfigs(opt::InputArgList &args) {
   ctx.arg.optimize = args::getInteger(args, OPT_O, 1);
   ctx.arg.outputFile = args.getLastArgValue(OPT_o);
   ctx.arg.relocatable = args.hasArg(OPT_relocatable);
+  ctx.arg.rpath = args::getStrings(args, OPT_rpath);
   ctx.arg.gcSections =
       args.hasFlag(OPT_gc_sections, OPT_no_gc_sections, !ctx.arg.relocatable);
   for (auto *arg : args.filtered(OPT_keep_section))

lld/wasm/Options.td

@@ -132,6 +132,8 @@ def relocatable: F<"relocatable">, HelpText<"Create relocatable object file">;
 
 defm reproduce: EEq<"reproduce", "Dump linker invocation and input files for debugging">;
 
+defm rpath: Eq<"rpath", "Add a DT_RUNPATH to the output">;
+
 defm rsp_quoting: Eq<"rsp-quoting", "Quoting style for response files">,
   MetaVarName<"[posix,windows]">;
 
@@ -289,6 +291,7 @@ def: Flag<["-"], "t">, Alias<trace>, HelpText<"Alias for --trace">;
 def: JoinedOrSeparate<["-"], "y">, Alias<trace_symbol>, HelpText<"Alias for --trace-symbol">;
 def: JoinedOrSeparate<["-"], "u">, Alias<undefined>;
 def: Flag<["-"], "V">, Alias<v>, HelpText<"Alias for -v">;
+def: JoinedOrSeparate<["-"], "R">, Alias<rpath>, HelpText<"Alias for --rpath">;
 
 // LTO-related options.
 def lto_O: JJ<"lto-O">, MetaVarName<"<opt-level>">,

lld/wasm/SyntheticSections.cpp

@@ -16,6 +16,7 @@
 #include "InputElement.h"
 #include "OutputSegment.h"
 #include "SymbolTable.h"
+#include "llvm/BinaryFormat/Wasm.h"
 #include "llvm/Support/Path.h"
 #include <optional>
 
@@ -133,6 +134,14 @@ void DylinkSection::writeBody() {
 
     sub.writeTo(os);
   }
+  if (!ctx.arg.rpath.empty()) {
+      SubSection sub(WASM_DYLINK_RUNTIME_PATH);
+      writeUleb128(sub.os, ctx.arg.rpath.size(), "num rpath entries");
+      for (const auto ref: ctx.arg.rpath) {
+        writeStr(sub.os, ref, "rpath entry");
+      }
+      sub.writeTo(os);
+  }
 }
 
 uint32_t TypeSection::registerType(const WasmSignature &sig) {