cmd/go: fix failing gccgo cases in TestScript/build_overlay

The 'go install' command does not support the -gccgo flag.
(I'm not sure why, but it doesn't.)

gccgo also uses system-native assembly syntax instead of cmd/compile's
Plan 9 derivative. I've added an assembly file that seems to work on
Linux, but I haven't tested it on other platforms; if it fails on
other platforms, we can refine the test as needed.

Fixes #42688

Change-Id: I0693a6a9eb58975f20cdc4160ef5f9a948563c88
Reviewed-on: https://go-review.googlesource.com/c/go/+/270978
Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

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

@@ -95,12 +95,6 @@ go build -compiler=gccgo -overlay overlay.json -o main_call_asm_gccgo$GOEXE ./ca
 exec ./main_call_asm_gccgo$GOEXE
 ! stdout .
 
-go install -gccgo -overlay overlay.json ./test_cache
-go list -gccgo -overlay overlay.json -f '{{.Stale}}' ./test_cache
-stdout '^false$'
-cp overlay/test_cache_different.go overlay/test_cache.go
-go list -gccgo -overlay overlay.json -f '{{.Stale}}' ./test_cache
-stdout '^true$'
 
 -- m/go.mod --
 // TODO(matloob): how do overlays work with go.mod (especially if mod=readonly)
@@ -128,7 +122,8 @@ the actual code is in the overlay
 		"dir2/i.go": "overlay/dir2_i.go",
 		"printpath/main.go": "overlay/printpath.go",
 		"printpath/other.go": "overlay2/printpath2.go",
-		"call_asm/asm.s": "overlay/asm_file.s",
+		"call_asm/asm_gc.s": "overlay/asm_gc.s",
+		"call_asm/asm_gccgo.s": "overlay/asm_gccgo.s",
 		"test_cache/main.go": "overlay/test_cache.go",
 		"cgo_hello_replace/cgo_header.h": "overlay/cgo_head.h",
 		"cgo_hello_replace/hello.c": "overlay/hello.c",
@@ -242,10 +237,20 @@ void say_hello();
 #include <stdio.h>
 
 void say_hello() { puts("hello cgo\n"); fflush(stdout); }
--- m/overlay/asm_file.s --
+-- m/overlay/asm_gc.s --
+// +build !gccgo
+
 TEXT ·foo(SB),0,$0
 	RET
 
+-- m/overlay/asm_gccgo.s --
+// +build gccgo
+
+.globl main.foo
+.text
+main.foo:
+	ret
+
 -- m/overlay/test_cache.go --
 package foo