libc: Add Scorpion-optimized memcpy and memset

 * Optimized routines from Qualcomm for msm8660 class SoCs
 * Enable with TARGET_CPU_VARIANT := scorpion
 * Use cortex-a15 memcpy and krait memset routines

Change-Id: I01d0f22efba5a418ddd20fca0d0c570d855e0f6f
libc: Add memchr for cortex-a53

Change-Id: Id84695006991bb9aaef8ddc127c3bee646948527