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/arch/x86/acpi.c

@@ -44,6 +44,7 @@
 #include <arch/acpigen.h>
 #include <device/pci.h>
 #include <cbmem.h>
+#include <compiler.h>
 #include <cpu/x86/lapic_def.h>
 #include <cpu/cpu.h>
 #include <cbfs.h>
@@ -971,7 +972,7 @@ void acpi_create_fadt(acpi_fadt_t *fadt, acpi_facs_t *facs, void *dsdt)
 }
 #endif
 
-unsigned long __attribute__((weak)) fw_cfg_acpi_tables(unsigned long start)
+unsigned long __weak fw_cfg_acpi_tables(unsigned long start)
 {
 	return 0;
 }
@@ -1238,7 +1239,7 @@ void acpi_save_gnvs(u32 gnvs_address)
 		*gnvs = gnvs_address;
 }
 
-__attribute__((weak)) int acpi_get_gpe(int gpe)
+__weak int acpi_get_gpe(int gpe)
 {
 	return -1; /* implemented by SOC */
 }