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/include/cpu/cpu.h

@@ -7,7 +7,7 @@
 void cpu_initialize(unsigned int cpu_index);
 struct bus;
 void initialize_cpus(struct bus *cpu_bus);
-void __attribute__((regparm(0))) secondary_cpu_init(unsigned int cpu_index);
+void asmlinkage secondary_cpu_init(unsigned int cpu_index);
 
 #if CONFIG_HAVE_SMI_HANDLER
 void smm_init(void);