libc: krait: Re-factor high-performance memcpy for thumb2

The majority of libc under bionic is built for thumb2.  Refactor the
high performance memcpy used in previous builds for thumb2, including
information that can be used for stack-unwinding.

Change-Id: Ib5f7ab354f39313758402ec02b0aea27b15d45fa

Conflicts:
	libc/arch-arm/krait/bionic/memcpy_base.S
(cherry picked from commit cdeb493351044c3116fc0d42a64f33f8215a5886)
(cherry picked from commit 0c524faa937b40178915c9478599b754b6f8f78d)