Revert "libm: Enable fast NEON pow() for supported targets"

 * Will revisit.

This reverts commit 1e126c23e21da1dcbefc6f5c444b97ef622e67ad.

Change-Id: Ie065e80f2230f0e1d22ed9f1242fe6eb6968d6df
libm: optimistic pow calling

Call optimized pow optimistically and revert to full range
implementation if we detect an out-of-range input.

Change-Id: I02657c2a254e9796e795a8fcc6ae43e24c9d37b9
libm: sqrt and sqrtf via ARM vsqrt instruction.

Optimized sqrt and sqrtf for arm by using hardware
opcode for sqrt rather than generic slow portable
code.

Change-Id: I9888d58547bcfca7dfc1400d3e7e222599167249