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/nvidia/tegra210/funitcfg.c b/src/soc/nvidia/tegra210/funitcfg.c
index a26da86..e8d0c97 100644
--- a/src/soc/nvidia/tegra210/funitcfg.c
+++ b/src/soc/nvidia/tegra210/funitcfg.c
@@ -14,6 +14,7 @@
  */
 
 #include <arch/io.h>
+#include <compiler.h>
 #include <soc/addressmap.h>
 #include <soc/clock.h>
 #include <soc/funitcfg.h>
@@ -175,7 +176,7 @@
 	}
 }
 
-void __attribute__((weak)) usb_setup_utmip(void *usb_base)
+void __weak usb_setup_utmip(void *usb_base)
 {
 	/* default empty implementation required if usb.c is not included */
 	printk(BIOS_ERR, "USB setup is not supported in current stage\n");