cmd/cgo: fix TestFortran on Windows

When gfortran is installed on Windows, there is no unversioned
libgfortran library - look for libgfortran.a instead.

This fixes a test failure introduced in CL 787800, which has not been
catched by the trybots because they don't have gfortran installed.

Change-Id: I8b8b87af4ca66c090691d950dc1b778c7e573af6
Reviewed-on: https://go-review.googlesource.com/c/go/+/791200
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/cgo/internal/testfortran/fortran_test.go

diff --git a/src/cmd/cgo/internal/testfortran/fortran_test.go b/src/cmd/cgo/internal/testfortran/fortran_test.go
index 1decd13..771590e 100644
--- a/src/cmd/cgo/internal/testfortran/fortran_test.go
+++ b/src/cmd/cgo/internal/testfortran/fortran_test.go
@@ -47,7 +47,7 @@
 		switch runtime.GOOS {
 		case "darwin":
 			libExt = "dylib"
-		case "aix", "openbsd":
+		case "aix", "openbsd", "windows":
 			libExt = "a"
 		}
 		libName := "libgfortran." + libExt