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/timer.c

@@ -13,12 +13,13 @@
  * GNU General Public License for more details.
  */
 
+#include <compiler.h>
 #include <console/console.h>
 #include <timer.h>
 #include <delay.h>
 #include <thread.h>
 
-__attribute__((weak)) void init_timer(void) { /* do nothing */ }
+__weak void init_timer(void) { /* do nothing */ }
 
 void udelay(unsigned int usec)
 {