cmd/go: fix TestIssue7573 for absolute paths in -L options

Updates #20266.
Fixes #20664.

Change-Id: Ifca30409fc7971497efb6d84f3f98760157c2233
Reviewed-on: https://go-review.googlesource.com/45650Reviewed-by: Austin Clements <austin@google.com>

src/cmd/go/go_test.go

@@ -2447,7 +2447,7 @@ import "C"
 func main() { C.f() }`)
 	tg.setenv("GOPATH", tg.path("."))
 	tg.run("build", "-n", "-compiler", "gccgo", "cgoref")
-	tg.grepStderr(`gccgo.*\-L alibpath \-lalib`, `no Go-inline "#cgo LDFLAGS:" ("-L alibpath -lalib") passed to gccgo linking stage`)
+	tg.grepStderr(`gccgo.*\-L [^ ]*alibpath \-lalib`, `no Go-inline "#cgo LDFLAGS:" ("-L alibpath -lalib") passed to gccgo linking stage`)
 }
 
 func TestListTemplateContextFunction(t *testing.T) {