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/device/pci_rom.c

@@ -19,6 +19,7 @@
 
 #include <console/console.h>
 #include <commonlib/endian.h>
+#include <compiler.h>
 #include <device/device.h>
 #include <device/pci.h>
 #include <device/pci_ids.h>
@@ -29,7 +30,7 @@
 #include <arch/acpigen.h>
 
 /* Rmodules don't like weak symbols. */
-u32 __attribute__((weak)) map_oprom_vendev(u32 vendev) { return vendev; }
+u32 __weak map_oprom_vendev(u32 vendev) { return vendev; }
 
 struct rom_header *pci_rom_probe(struct device *dev)
 {