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/amd/agesa/romstage.c b/src/drivers/amd/agesa/romstage.c
index bd502b6..197a007 100644
--- a/src/drivers/amd/agesa/romstage.c
+++ b/src/drivers/amd/agesa/romstage.c
@@ -18,6 +18,7 @@
 #include <cbmem.h>
 #include <cpu/amd/car.h>
 #include <cpu/x86/bist.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <halt.h>
 #include <program_loading.h>
@@ -43,7 +44,7 @@
 	amd_initmmio();
 }
 
-void __attribute__((weak)) platform_once(struct sysinfo *cb)
+void __weak platform_once(struct sysinfo *cb)
 {
 	board_BeforeAgesa(cb);
 }