runtime/cgo: restore correct build tags for linux/mips*

The linux build tags were incorrectly removed from these files by CL 460538.
Restore the correct build tags so that they are only included in builds
for linux/mips* platforms.

Change-Id: I21d8802b0252688d8e2228cf904b47d90b253485
Reviewed-on: https://go-review.googlesource.com/c/go/+/469175
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/runtime/cgo/gcc_linux_mips64x.c

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build mips64 || mips64le
+//go:build linux && (mips64 || mips64le)
 
 #include <pthread.h>
 #include <string.h>

src/runtime/cgo/gcc_linux_mipsx.c

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build mips || mipsle
+//go:build linux && (mips || mipsle)
 
 #include <pthread.h>
 #include <string.h>