Kbuild: allow arch/xxx to use a different source path

Preparatory patch for the source merge of arch/i386 and arch/x86_64
into arch/x86. This allows to keep the original arch directories as
stubs for the main Makefiles, Kconfigs et. al during the transition
phase while having the code in the new arch/x86 directory.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>