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/baytrail/southcluster.c

@@ -19,6 +19,7 @@
 #include <arch/acpi.h>
 #include <bootstate.h>
 #include <cbmem.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <cpu/x86/smm.h>
 #include <device/device.h>
@@ -536,7 +537,7 @@ static const struct pci_driver southcluster __pci_driver = {
 	.device		= LPC_DEVID,
 };
 
-int __attribute__((weak)) mainboard_get_spi_config(struct spi_config *cfg)
+int __weak mainboard_get_spi_config(struct spi_config *cfg)
 {
 	return -1;
 }