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>
diff --git a/src/mainboard/google/kahlee/variants/baseboard/memory.c b/src/mainboard/google/kahlee/variants/baseboard/memory.c
index b8ec917..280140b 100644
--- a/src/mainboard/google/kahlee/variants/baseboard/memory.c
+++ b/src/mainboard/google/kahlee/variants/baseboard/memory.c
@@ -14,13 +14,14 @@
  */
 
 #include <baseboard/variants.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <gpio.h> /* src/include/gpio.h */
 #include <spd_bin.h>
 #include <variant/gpio.h>
 #include <amdblocks/dimm_spd.h>
 
-uint8_t __attribute__((weak)) variant_memory_sku(void)
+uint8_t __weak variant_memory_sku(void)
 {
 	gpio_t pads[] = {
 		[3] = MEM_CONFIG3,
@@ -32,7 +33,7 @@
 	return gpio_base2_value(pads, ARRAY_SIZE(pads));
 }
 
-int __attribute__((weak)) variant_mainboard_read_spd(uint8_t spdAddress,
+int __weak variant_mainboard_read_spd(uint8_t spdAddress,
 							char *buf, size_t len)
 {
 	struct region_device spd_rdev;