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>
diff --git a/src/drivers/intel/fsp2_0/notify.c b/src/drivers/intel/fsp2_0/notify.c
index 34a4262..6d6d623 100644
--- a/src/drivers/intel/fsp2_0/notify.c
+++ b/src/drivers/intel/fsp2_0/notify.c
@@ -12,6 +12,7 @@
 
 #include <arch/cpu.h>
 #include <bootstate.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <fsp/util.h>
 #include <soc/intel/common/util.h>
@@ -87,7 +88,7 @@
 BOOT_STATE_INIT_ENTRY(BS_OS_RESUME, BS_ON_ENTRY, fsp_notify_dummy,
 						(void *) READY_TO_BOOT);
 
-__attribute__((weak)) void platform_fsp_notify_status(
+__weak void platform_fsp_notify_status(
 	enum fsp_notify_phase phase)
 {
 }