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/braswell/chip.c

@@ -15,6 +15,7 @@
  */
 
 #include <chip.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <device/device.h>
 #include <device/pci.h>
@@ -81,7 +82,7 @@ static void enable_dev(device_t dev)
 	}
 }
 
-__attribute__((weak)) void board_silicon_USB2_override(SILICON_INIT_UPD *params)
+__weak void board_silicon_USB2_override(SILICON_INIT_UPD *params)
 {
 }