Break bionic implementations into arch versions.

Move arch specific code for arm, mips, x86 into separate
makefiles.
In addition, add different arm cpu versions of memcpy/memset.

Bug: 8005082

Merge from internal master (acdde8c1cf8e8beed98c052757d96695b820b50c).

Change-Id: I04f3d0715104fab618e1abf7cf8f7eec9bec79df
Merge "Break bionic implementations into arch versions."
Config: Add CPU Variant option
Merge remote-tracking branch 'aosp/master'
Move ARM specified build rule to libc/arch-arm/arm.mk
Config: Make output directory configurable
Merge branch 'master' of https://github.com/olibc/olibc