compiler.h: add __weak macro

Instead of writing out '__attribute__((weak))' use a shorter form.

Change-Id: If418a1d55052780077febd2d8f2089021f414b91
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25767
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Justin TerAvest <teravest@chromium.org>

src/ec/google/chromeec/crosec_proto.c

@@ -14,6 +14,7 @@
  */
 
 #include <arch/io.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <delay.h>
 #include <stdint.h>
@@ -23,7 +24,7 @@
 #include "ec_message.h"
 
 /* Common utilities */
-void * __attribute__((weak)) crosec_get_buffer(size_t size, int req)
+void * __weak crosec_get_buffer(size_t size, int req)
 {
 	printk(BIOS_DEBUG, "crosec_get_buffer() implementation required.\n");
 	return NULL;