AGESA: Move romstage main entry under cpu

As we now apply asmlinkage attributes to romstage_main()
entry, also x86_64 passes parameters on the stack.

Change-Id: If9938dbbe9a164c9c1029431499b51ffccb459c1
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/18624
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/cpu/amd/car.h

@@ -18,4 +18,6 @@ void disable_cache_as_ram(void);
 
 void asmlinkage early_all_cores(void);
 
+void * asmlinkage romstage_main(unsigned long bist);
+
 #endif