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/drivers/i2c/tpm/cr50.c

@@ -31,6 +31,7 @@
 
 #include <arch/early_variables.h>
 #include <commonlib/endian.h>
+#include <compiler.h>
 #include <stdint.h>
 #include <string.h>
 #include <types.h>
@@ -58,7 +59,7 @@ struct tpm_inf_dev {
 
 static struct tpm_inf_dev g_tpm_dev CAR_GLOBAL;
 
-__attribute__((weak)) int tis_plat_irq_status(void)
+__weak int tis_plat_irq_status(void)
 {
 	static int warning_displayed CAR_GLOBAL;