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/apollolake/chip.c

@@ -20,6 +20,7 @@
 #include <arch/acpi.h>
 #include <bootstate.h>
 #include <cbmem.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <cpu/cpu.h>
 #include <cpu/x86/mp.h>
@@ -518,7 +519,7 @@ static void glk_fsp_silicon_init_params_cb(
 		sizeof(silconfig->PcieRpSelectableDeemphasis));
 }
 
-void __attribute__((weak)) mainboard_devtree_update(struct device *dev)
+void __weak mainboard_devtree_update(struct device *dev)
 {
        /* Override dev tree settings per board */
 }