cmd/go: make sure linker -o for plugin doesn't include tempdir path

There is already a case that when buildmode=shared passes only the
basename of the -o argument to the link command to the linker (and
runs in the directory of that argument) to avoid having that
(temporary) directory of the file be included in the LC_ID_DYLIB load
command. Extend the case to buildmode=plugin, because the same thing
can happen there.

This can only happen on darwin: the -o command can be embedded into
Mach-O and PE binaries, but plugin isn't supported on Windows.

For #58557

Change-Id: I7a4a5627148e77c6906ac4583af3d9f053d5b249
Reviewed-on: https://go-review.googlesource.com/c/go/+/477296
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/work/gc.go

@@ -684,7 +684,7 @@ func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string)
 	// On Windows, DLL file name is recorded in PE file
 	// export section, so do like on OS X.
 	dir := "."
-	if (cfg.Goos == "darwin" || cfg.Goos == "windows") && cfg.BuildBuildmode == "c-shared" {
+	if (cfg.Goos == "darwin" || cfg.Goos == "windows") && (cfg.BuildBuildmode == "c-shared" || cfg.BuildBuildmode == "plugin") {
 		dir, out = filepath.Split(out)
 	}
 

src/cmd/go/testdata/script/build_plugin_reproducible.txt

@@ -0,0 +1,11 @@
+[!buildmode:plugin] skip
+[short] skip
+
+go build -trimpath -buildvcs=false -buildmode=plugin -o a.so main.go
+go build -trimpath -buildvcs=false -buildmode=plugin -o b.so main.go
+cmp a.so b.so
+
+-- main.go --
+package main
+
+func main() {}