AndroidKernel.mk: Parameterize ARCH and CROSS_COMPILE compile flags

Allow overriding of the ARCH and CROSS_COMPILE parameters passed
to kernel make, with fallback to previous hardcoded values if
the overrides are not defined.

Change-Id: I33b571e22363fdc06eb9b867e13d4487a7b5bf55
Signed-off-by: David Ng <dave@codeaurora.org>