1. Moved arch-specific setup to their own files:
- <arch>/<arch>.mk, arch-specific configs. Variables in those config
end with the arch name.
- removed the extra complexity introduced by function libc-add-cpu-variant-src,
which seems to be not very useful these days.
2. Separated out the crt object files generation rules and set up the
rules for both TARGET_ARCH and TARGET_2ND_ARCH.
3. Build all the libraries for both TARGET_ARCH and TARGET_2ND_ARCH,
with the arch-specific LOCAL_ variables.
Bug: 11654773
Change-Id: I9c2d85db0affa49199d182236d2210060a321421
