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/lib/imd_cbmem.c

@@ -15,6 +15,7 @@
 
 #include <bootstate.h>
 #include <bootmem.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <cbmem.h>
 #include <imd.h>
@@ -109,7 +110,7 @@ void cbmem_initialize_empty(void)
 	cbmem_initialize_empty_id_size(0, 0);
 }
 
-void __attribute__((weak)) cbmem_top_init(void)
+void __weak cbmem_top_init(void)
 {
 }