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/arch/arm/stages.c

@@ -26,11 +26,12 @@
 
 #include <arch/stages.h>
 #include <arch/cache.h>
+#include <compiler.h>
 
 /**
  * generic stage entry point. override this if board specific code is needed.
  */
-__attribute__((weak)) void stage_entry(void)
+__weak void stage_entry(void)
 {
 	main();
 }