Example: for a file asm/foo.c, make will look for asm/arm/foo.[cS] and
compile it if found. If not found it'll fall back to asm/foo.c.
Also introduce new ARCH make variable. This is automatically detected by
configure. It is distinct from CPU since CPU defines the dir used for
the target tree (i.e. firmware/target/X, so it can be "hosted").
ARCH really has the target isa and can be x86 for sims/raaa too.
Change-Id: I18e5d2b7b7bbc2ad2be551a74a0fcae5ffbcbf8b
