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/lib/bootblock.c b/src/lib/bootblock.c
index 2e228c6..bee2845 100644
--- a/src/lib/bootblock.c
+++ b/src/lib/bootblock.c
@@ -16,6 +16,7 @@
 
 #include <arch/exception.h>
 #include <bootblock_common.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <delay.h>
 #include <pc80/mc146818rtc.h>
@@ -25,10 +26,10 @@
 
 DECLARE_OPTIONAL_REGION(timestamp);
 
-__attribute__((weak)) void bootblock_mainboard_early_init(void) { /* no-op */ }
-__attribute__((weak)) void bootblock_soc_early_init(void) { /* do nothing */ }
-__attribute__((weak)) void bootblock_soc_init(void) { /* do nothing */ }
-__attribute__((weak)) void bootblock_mainboard_init(void) { /* do nothing */ }
+__weak void bootblock_mainboard_early_init(void) { /* no-op */ }
+__weak void bootblock_soc_early_init(void) { /* do nothing */ }
+__weak void bootblock_soc_init(void) { /* do nothing */ }
+__weak void bootblock_mainboard_init(void) { /* do nothing */ }
 
 asmlinkage void bootblock_main_with_timestamp(uint64_t base_timestamp)
 {