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/braswell/acpi.c

@@ -22,6 +22,7 @@
 #include <arch/smp/mpspec.h>
 #include <cbfs.h>
 #include <cbmem.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <cpu/cpu.h>
 #include <cpu/intel/turbo.h>
@@ -552,6 +553,6 @@ void southcluster_inject_dsdt(device_t device)
 	}
 }
 
-__attribute__((weak)) void acpi_create_serialio_ssdt(acpi_header_t *ssdt)
+__weak void acpi_create_serialio_ssdt(acpi_header_t *ssdt)
 {
 }