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/soc/intel/common/vbt.c

@@ -14,6 +14,7 @@
  */
 
 #include <cbfs.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <arch/acpi.h>
 #include <bootmode.h>
@@ -23,7 +24,7 @@
 
 #define VBT_SIGNATURE 0x54425624
 
-__attribute__((weak))
+__weak
 const char *mainboard_vbt_filename(void)
 {
 	return "vbt.bin";