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/console/post.c

@@ -16,6 +16,7 @@
 
 #include <arch/io.h>
 #include <elog.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <device/device.h>
 #include <pc80/mc146818rtc.h>
@@ -29,7 +30,7 @@
 /* Some mainboards have very nice features beyond just a simple display.
  * They can override this function.
  */
-void __attribute__((weak)) mainboard_post(uint8_t value)
+void __weak mainboard_post(uint8_t value)
 {
 }