cmd/internal/objfile: recognize Windows ARM64 executables

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.

Change-Id: I5ec459063d394b9f434d1b8b5030960b45061038
Reviewed-on: https://go-review.googlesource.com/c/go/+/288821
Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/cmd/internal/objfile/pe.go

@@ -189,6 +189,8 @@ func (f *peFile) goarch() string {
 		return "amd64"
 	case pe.IMAGE_FILE_MACHINE_ARMNT:
 		return "arm"
+	case pe.IMAGE_FILE_MACHINE_ARM64:
+		return "arm64"
 	default:
 		return ""
 	}