internal/goroot: for -compiler=gccgo, only look for .gox files

Fixes #34358

Change-Id: I3022ac88e1ad595dc72c0063852b8d4a1ec3f0ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/196120
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -0,0 +1,15 @@
+# It's absurd, but builds with -compiler=gccgo used to fail to build module m.
+# golang.org/issue/34358
+
+cd m
+go build
+exists m$GOEXE
+rm m$GOEXE
+[exec:gccgo] go build -compiler=gccgo
+[exec:gccgo] exists m$GOEXE
+
+-- m/go.mod --
+module m
+-- m/main.go --
+package main
+func main() {}

src/internal/goroot/gc.go

@@ -121,18 +121,9 @@ func (gd *gccgoDirs) isStandard(path string) bool {
 	}
 
 	for _, dir := range gd.dirs {
-		full := filepath.Join(dir, path)
-		pkgdir, pkg := filepath.Split(full)
-		for _, p := range [...]string{
-			full,
-			full + ".gox",
-			pkgdir + "lib" + pkg + ".so",
-			pkgdir + "lib" + pkg + ".a",
-			full + ".o",
-		} {
-			if fi, err := os.Stat(p); err == nil && !fi.IsDir() {
-				return true
-			}
+		full := filepath.Join(dir, path) + ".gox"
+		if fi, err := os.Stat(full); err == nil && !fi.IsDir() {
+			return true
 		}
 	}