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/security/vboot/vboot_common.c

@@ -16,6 +16,7 @@
 #include <boot/coreboot_tables.h>
 #include <boot_device.h>
 #include <cbmem.h>
+#include <compiler.h>
 #include <console/cbmem_console.h>
 #include <console/console.h>
 #include <fmap.h>
@@ -101,7 +102,7 @@ int vboot_handoff_get_recovery_reason(void)
 }
 
 /* ============================ VBOOT REBOOT ============================== */
-void __attribute__((weak)) vboot_platform_prepare_reboot(void)
+void __weak vboot_platform_prepare_reboot(void)
 {
 }