cmd/cgo: use --no-gc-sections if available

zig cc passes `--gc-sections` to the underlying linker, which then
causes undefined symbol errors when compiling with cgo but without C
code. Add `-Wl,--no-gc-sections` to make it work with zig cc. Minimal
example:

**main.go**

    package main
    import _ "runtime/cgo"
    func main() {}

Run (works after the patch, doesn't work before):

    CC="zig cc" go build main.go

Among the existing code, `src/runtime/testdata/testprognet` fails to
build:

    src/runtime/testdata/testprognet$ CC="zig cc" go build .
    net(.text): relocation target __errno_location not defined
    net(.text): relocation target getaddrinfo not defined
    net(.text): relocation target freeaddrinfo not defined
    net(.text): relocation target gai_strerror not defined
    runtime/cgo(.text): relocation target stderr not defined
    runtime/cgo(.text): relocation target fwrite not defined
    runtime/cgo(.text): relocation target vfprintf not defined
    runtime/cgo(.text): relocation target fputc not defined
    runtime/cgo(.text): relocation target abort not defined
    runtime/cgo(.text): relocation target pthread_create not defined
    runtime/cgo(.text): relocation target nanosleep not defined
    runtime/cgo(.text): relocation target pthread_detach not defined
    runtime/cgo(.text): relocation target stderr not defined
    runtime/cgo(.text): relocation target strerror not defined
    runtime/cgo(.text): relocation target fprintf not defined
    runtime/cgo(.text): relocation target abort not defined
    runtime/cgo(.text): relocation target pthread_mutex_lock not defined
    runtime/cgo(.text): relocation target pthread_cond_wait not defined
    runtime/cgo(.text): relocation target pthread_mutex_unlock not defined
    runtime/cgo(.text): relocation target pthread_cond_broadcast not defined
    runtime/cgo(.text): relocation target malloc not defined

With the patch both examples build as expected.

@ianlancetaylor suggested:

> It would be fine with me if somebody wants to send a cgo patch that
passes -Wl,--no-gc-sections, with a fallback if that option is not
supported.

... and this is what we are doing. Tested with zig
0.10.0-dev.2252+a4369918b

Fixes #52690

Change-Id: Ib6d1b2bd59335e9663afefd360ddad7da358a938
GitHub-Last-Rev: 58406b3
GitHub-Pull-Request: #52815
Reviewed-on: https://go-review.googlesource.com/c/go/+/405414
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: motiejus

src/cmd/go/internal/work/exec.go

@@ -2528,6 +2528,13 @@ func (b *Builder) compilerCmd(compiler []string, incdir, workdir string) []strin
 		a = append(a, "-Qunused-arguments")
 	}
 
+	// zig cc passes --gc-sections to the underlying linker, which then causes
+	// undefined symbol errors when compiling with cgo but without C code.
+	// https://github.com/golang/go/issues/52690
+	if b.gccSupportsFlag(compiler, "-Wl,--no-gc-sections") {
+		a = append(a, "-Wl,--no-gc-sections")
+	}
+
 	// disable word wrapping in error messages
 	a = append(a, "-fmessage-length=0")
 
@@ -2594,13 +2601,21 @@ func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
 		defer os.Remove(tmp)
 	}
 
-	// We used to write an empty C file, but that gets complicated with
-	// go build -n. We tried using a file that does not exist, but that
-	// fails on systems with GCC version 4.2.1; that is the last GPLv2
-	// version of GCC, so some systems have frozen on it.
-	// Now we pass an empty file on stdin, which should work at least for
-	// GCC and clang.
-	cmdArgs := str.StringList(compiler, flag, "-c", "-x", "c", "-", "-o", tmp)
+	// We used to write an empty C file, but that gets complicated with go
+	// build -n. We tried using a file that does not exist, but that fails on
+	// systems with GCC version 4.2.1; that is the last GPLv2 version of GCC,
+	// so some systems have frozen on it. Now we pass an empty file on stdin,
+	// which should work at least for GCC and clang.
+	//
+	// If the argument is "-Wl,", then it's testing the linker. In that case,
+	// skip "-c". If it's not "-Wl,", then we are testing the compiler and
+	// can emit the linking step with "-c".
+	cmdArgs := str.StringList(compiler, flag)
+	if !strings.HasPrefix(flag, "-Wl,") /* linker flag */ {
+		cmdArgs = append(cmdArgs, "-c")
+	}
+	cmdArgs = append(cmdArgs, "-x", "c", "-", "-o", tmp)
+
 	if cfg.BuildN || cfg.BuildX {
 		b.Showcmd(b.WorkDir, "%s || true", joinUnambiguously(cmdArgs))
 		if cfg.BuildN {