libc: Add ARMV7 optimized memchr()

 * From Cortex-Strings via Linaro
 * Performance is 10x over the default version on microbenchmarks

Change-Id: I14c1b5ccbd0078219a1dbf9f02fb1591b3a2403c

Conflicts:
	libc/Android.mk
Bionic/libm: Fix the mismatch ENTRY/END in e_pow.S

Change-Id: Ibe53e7863f462a63029d1154f317256c00814c6d