haswell: use asmlinkage for assembly-called funcs

When the haswell MP/SMM code was developed it was using a coreboot
repository that did not contain the asmlinkage macro. Now that the
asmlinkage macro exists use it.

BUG=None
BRANCH=None
TEST=Built and booted.

Change-Id: I662f1b16d1777263b96a427334fff8f98a407755
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/3203
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/src/cpu/intel/haswell/romstage.c b/src/cpu/intel/haswell/romstage.c
index ff57584..077e409 100644
--- a/src/cpu/intel/haswell/romstage.c
+++ b/src/cpu/intel/haswell/romstage.c
@@ -162,7 +162,7 @@
 	return slot;
 }
 
-void * __attribute__((regparm(0))) romstage_main(unsigned long bist)
+void * asmlinkage romstage_main(unsigned long bist)
 {
 	int i;
 	void *romstage_stack_after_car;