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/soc/intel/common/block/pmc/pmc.c b/src/soc/intel/common/block/pmc/pmc.c
index d3fef55..c8e8026 100644
--- a/src/soc/intel/common/block/pmc/pmc.c
+++ b/src/soc/intel/common/block/pmc/pmc.c
@@ -15,6 +15,7 @@
 
 #include <arch/acpi.h>
 #include <arch/io.h>
+#include <compiler.h>
 #include <console/console.h>
 #include <cpu/x86/smm.h>
 #include <device/pci.h>
@@ -25,7 +26,7 @@
 /* SoC overrides */
 
 /* Fill up PMC resource structure inside SoC directory */
-__attribute__((weak)) int pmc_soc_get_resources(
+__weak int pmc_soc_get_resources(
 		struct pmc_resource_config *cfg)
 {
 	/* no-op */
@@ -33,7 +34,7 @@
 }
 
 /* SoC override PMC initialization */
-__attribute__((weak)) void pmc_soc_init(struct device *dev)
+__weak void pmc_soc_init(struct device *dev)
 {
 	/* no-op */
 }