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/skylake/chip_fsp20.c

@@ -20,6 +20,7 @@
 #include <fsp/api.h>
 #include <arch/acpi.h>
 #include <chip.h>
+#include <compiler.h>
 #include <bootstate.h>
 #include <console/console.h>
 #include <device/device.h>
@@ -346,7 +347,7 @@ void platform_fsp_silicon_init_params_cb(FSPS_UPD *supd)
 }
 
 /* Mainboard GPIO Configuration */
-__attribute__((weak)) void mainboard_silicon_init_params(FSP_S_CONFIG *params)
+__weak void mainboard_silicon_init_params(FSP_S_CONFIG *params)
 {
 	printk(BIOS_DEBUG, "WEAK: %s/%s called\n", __FILE__, __func__);
 }