x86: create smpcommon.c

This patch creates smpcommon.c with functions that are
equal between architectures. The i386-only init_gdt
is ifdef'd.

Note that smpcommon.o figures twice in the Makefile:
this is because sub-architectures like voyager that does
not use the normal smp_$(BITS) files also have to access them

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>