libc: add TARGET_CPU_VARIANT compatible for tegra2

Tegra2 is a Dual Cortex A9 without FP neon SIMD

based on 'generic' arm (memcpy, memset)
on cortex-a15 and a8 (strcmp)
and armv7-a from cm10.1 (memchr and strlen)

Change-Id: I1e7207e2d1ba02012fb5306f46b688903d31386c
Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>