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/security/vboot/bootmode.c

@@ -17,6 +17,7 @@
 #include <bootmode.h>
 #include <bootstate.h>
 #include <cbmem.h>
+#include <compiler.h>
 #include <rules.h>
 #include <string.h>
 #include <vb2_api.h>
@@ -141,18 +142,18 @@ int vboot_recovery_mode_enabled(void)
 	return !!vboot_check_recovery_request();
 }
 
-int __attribute__((weak)) clear_recovery_mode_switch(void)
+int __weak clear_recovery_mode_switch(void)
 {
 	// Weak implementation. Nothing to do.
 	return 0;
 }
 
-void __attribute__((weak)) log_recovery_mode_switch(void)
+void __weak log_recovery_mode_switch(void)
 {
 	// Weak implementation. Nothing to do.
 }
 
-int __attribute__((weak)) get_recovery_mode_retrain_switch(void)
+int __weak get_recovery_mode_retrain_switch(void)
 {
 	return 0;
 }
@@ -175,12 +176,12 @@ int vboot_developer_mode_enabled(void)
  * TODO: Create flash protection interface which implements get_write_protect_state.
  * get_recovery_mode_switch should be implemented as default function.
  */
-int __attribute__((weak)) get_write_protect_state(void)
+int __weak get_write_protect_state(void)
 {
 	return 0;
 }
 
-int __attribute__((weak)) get_recovery_mode_switch(void)
+int __weak get_recovery_mode_switch(void)
 {
 	return 0;
 }