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/gpio.c

@@ -14,6 +14,7 @@
  */
 
 #include <device/pci.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <soc/gpio.h>
 #include <soc/pmc.h>
@@ -234,7 +235,7 @@ void setup_soc_gpios(struct soc_gpio_config *config, u8 enable_xdp_tap)
 	}
 }
 
-struct soc_gpio_config* __attribute__((weak)) mainboard_get_gpios(void)
+struct soc_gpio_config* __weak mainboard_get_gpios(void)
 {
 	printk(BIOS_DEBUG, "Default/empty GPIO config\n");
 	return NULL;