libm: Enable fast NEON pow() for supported targets

Change-Id: Iabda43e3c8462074a9a46c8f496eac903ea17084
libc: Add ARCH_ARM flags to enable new optimizations

Change-Id: I2a436f0730201509c6e53da7447835653d75f7f5

Conflicts:

	libc/Android.mk