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/arch/x86/pirq_routing.c

@@ -15,12 +15,13 @@
  * GNU General Public License for more details.
  */
 #include <console/console.h>
+#include <compiler.h>
 #include <arch/pirq_routing.h>
 #include <string.h>
 #include <device/pci.h>
 #include <arch/pirq_routing.h>
 
-void __attribute__((weak)) pirq_assign_irqs(const unsigned char pirq[CONFIG_MAX_PIRQ_LINKS])
+void __weak pirq_assign_irqs(const unsigned char pirq[CONFIG_MAX_PIRQ_LINKS])
 {
 }