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/drivers/intel/fsp1_1/car.c

@@ -14,6 +14,7 @@
  */
 
 #include <arch/early_variables.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <fsp/car.h>
 #include <fsp/util.h>
@@ -100,18 +101,18 @@ asmlinkage void after_cache_as_ram(void *chipset_context)
 	after_cache_as_ram_stage();
 }
 
-void __attribute__((weak)) car_mainboard_pre_console_init(void)
+void __weak car_mainboard_pre_console_init(void)
 {
 }
 
-void __attribute__((weak)) car_soc_pre_console_init(void)
+void __weak car_soc_pre_console_init(void)
 {
 }
 
-void __attribute__((weak)) car_mainboard_post_console_init(void)
+void __weak car_mainboard_post_console_init(void)
 {
 }
 
-void __attribute__((weak)) car_soc_post_console_init(void)
+void __weak car_soc_post_console_init(void)
 {
 }