libc: use Cortex-A7/A53 memset on Kryo

Tested on OnePlus 3 with MSM8996

Before (Krait):
BM_string_memset/8                          1000k          7    1.071 GiB/s
BM_string_memset/64                         1000k          9    6.720 GiB/s
BM_string_memset/512                        1000k         19   26.699 GiB/s
BM_string_memset/1024                       1000k         29   34.214 GiB/s
BM_string_memset/8Ki                          10M        184   44.302 GiB/s
BM_string_memset/16Ki                          5M        351   46.656 GiB/s
BM_string_memset/32Ki                          5M        686   47.744 GiB/s
BM_string_memset/64Ki                       1000k       1355   48.342 GiB/s

After (A7/A53):
BM_string_memset/8                          1000k          7    1.017 GiB/s
BM_string_memset/64                         1000k          7    8.039 GiB/s
BM_string_memset/512                        1000k         13   37.976 GiB/s
BM_string_memset/1024                       1000k         21   47.595 GiB/s
BM_string_memset/8Ki                          10M        150   54.354 GiB/s
BM_string_memset/16Ki                         10M        285   57.336 GiB/s
BM_string_memset/32Ki                          5M        555   59.016 GiB/s
BM_string_memset/64Ki                       1000k       1097   59.720 GiB/s

Test: make otapackage
Test: Verified this wins in all dimensions on a Pixel device.

Change-Id: I4cb72cc0363877de7c41d6f991149fda42fd0b4b