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/quark/gpio_i2c.c

@@ -13,6 +13,7 @@
  * GNU General Public License for more details.
  */
 
+#include <compiler.h>
 #include <console/console.h>
 #include <delay.h>
 #include <device/device.h>
@@ -23,7 +24,7 @@
 #include <soc/ramstage.h>
 #include <soc/reg_access.h>
 
-__attribute__((weak)) void mainboard_gpio_i2c_init(device_t dev)
+__weak void mainboard_gpio_i2c_init(device_t dev)
 {
 	/* Initialize any of the GPIOs or I2C devices */
 	printk(BIOS_SPEW, "WEAK; mainboard_gpio_i2c_init\n");