math: new powf

math: new powf

from https://github.com/ARM-software/optimized-routines,
commit 04884bd04eac4b251da4026900010ea7d8850edc

POWF_SCALE != 1.0 case only matters if TOINT_INTRINSICS is set, which
is currently not supported for any target.

SNaN is not supported, it would require an issignalingf
implementation.

code size change: -816 bytes.
benchmark on x86_64 before, after, speedup:

-Os:
  powf rthruput:  95.14 ns/call 20.04 ns/call 4.75x
   powf latency: 137.00 ns/call 34.98 ns/call 3.92x
-O3:
  powf rthruput:  92.48 ns/call 13.67 ns/call 6.77x
   powf latency: 131.11 ns/call 35.15 ns/call 3.73x