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/lib/fallback_boot.c

@@ -1,8 +1,9 @@
 #include <fallback.h>
+#include <compiler.h>
 #include <watchdog.h>
 
 /* Implement platform specific override. */
-void __attribute__((weak)) set_boot_successful(void) { }
+void __weak set_boot_successful(void) { }
 
 void boot_successful(void)
 {