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/soc/intel/common/block/sram/sram.c b/src/soc/intel/common/block/sram/sram.c
index 49367e1..9028952 100644
--- a/src/soc/intel/common/block/sram/sram.c
+++ b/src/soc/intel/common/block/sram/sram.c
@@ -14,13 +14,14 @@
  * GNU General Public License for more details.
  */
 
+#include <compiler.h>
 #include <device/device.h>
 #include <device/pci.h>
 #include <device/pci_ids.h>
 #include <intelblocks/sram.h>
 #include <soc/iomap.h>
 
-__attribute__((weak)) void soc_sram_init(struct device *dev) { /* no-op */ }
+__weak void soc_sram_init(struct device *dev) { /* no-op */ }
 
 static void sram_read_resources(struct device *dev)
 {