arch: Ignore arch/i386 and arch/x86_64

The i386 and x86_64 arch directories contain nothing but a generated symlink
to arch/x86/boot/bzImage when a tree a built.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Anvin <hpa@zytor.com>
Signed-off-by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>