x86: move i386 and x86_64 Makefiles to arch/x86

Moving the ARCH specific Makefiles for i386 and x86_64
required a litle bit tweaking in the top-lvel Makefile.

SRCARCH is now set in the top-level Makefile
because we need this info to include the correct
arch Makefile.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>