Unify assembler function handling

Instead of adding regparm(0) to each assembler function called
by coreboot, add an asmlinkage macro (like the Linux kernel does)
that can be different per architecture (and that is  empty on ARM
right now)

Change-Id: I7ad10c463f6c552f1201f77ae24ed354ac48e2d9
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1973
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/x86/boot/acpi.c

@@ -748,7 +748,7 @@ extern int lowmem_backup_size;
 #define WAKEUP_BASE 0x600
 
 void (*acpi_do_wakeup)(u32 vector, u32 backup_source, u32 backup_target,
-       u32 backup_size) __attribute__((regparm(0))) = (void *)WAKEUP_BASE;
+       u32 backup_size) asmlinkage = (void *)WAKEUP_BASE;
 
 extern unsigned char __wakeup, __wakeup_size;