Enable 32-bit armv8 NEON for libm

Since ARM neon instructions were only used on armv7-a-neon architecture
variant, the default implementation for 32-bit armv8-a cores doesn't
use these advanced SIMD instructions. By using "neon" key in the
Android.bp, both arch variants (armv7-a-neon and armv8-a) are covered.

Bug: 65569003
Test: lunch aosp_arm64; emulator # on oc-mr1-dev; boot to home screen
      Also checked if sqrt.o and floor.o are actually built and linked
      in the resulted libm.a for both armv7-a-neon/cortex-a15 and
      armv8-a/generic (2nd) arch/cpu variant.
Change-Id: I2084dbdb12e252b06bba5adc65adb59e97a99332