cmd/go: make sure the linker for shared doesn't include tempdir path

This is similar to CL 478196 and CL 477296,
but this is for -buildmode=shared.

When using "go install -buildmode=shared std",
because the gold linker is used by default on Linux arm64,
it will cause temporary paths to be included in libstd.so.

Based on the changes of CL 478196,
I speculate that this may also have issues on other platforms.
So, this change is for all platform.

But I don't have any other platforms and
don't know what the file name for verification,
so the test cases are only for the Linux platform.
I hope someone can improve this testcase. Thanks!

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

@@ -714,7 +714,10 @@ func (gcToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action,
 		}
 		ldflags = append(ldflags, d.Package.ImportPath+"="+d.Target)
 	}
-	return b.Shell(root).run(".", targetPath, nil, cfg.BuildToolexec, base.Tool("link"), "-o", targetPath, "-importcfg", importcfg, ldflags)
+
+	// Same reason in (*gcToolchain).ld
+	dir, targetPath := filepath.Split(targetPath)
+	return b.Shell(root).run(dir, targetPath, nil, cfg.BuildToolexec, base.Tool("link"), "-o", targetPath, "-importcfg", importcfg, ldflags)
 }
 
 func (gcToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {

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

@@ -0,0 +1,10 @@
+[!buildmode:shared] skip
+[short] skip
+[!cgo] skip '-buildmode=shared requires external linking'
+[!GOOS:linux] skip
+
+env GO111MODULE=off
+env CGO_ENABLED=1
+go install -a -trimpath -buildvcs=false -buildmode=shared -pkgdir=pkgdir1 runtime
+go install -a -trimpath -buildvcs=false -buildmode=shared -pkgdir=pkgdir2 runtime
+[GOOS:linux] cmp -q pkgdir1/libruntime.so pkgdir2/libruntime.so