one time is enough
Bionic/libm: Pow optimizations and bug fixes.

Use VFP calling convention for pow_neon handoff function by default.
Fix register usage collision between two different polynomial
coefficients in pow_neon.  Remove conditional execution in pow_neon
and replace with branching.

Change-Id: I76095f4a006e2fb01a53943b66fd69bfa1fd3033
(cherry picked from commit 5f0ee8c38363ecb99f7ceb487583b5e7152e63ce)
(cherry picked from commit f4a020fad7881195bff087b5803e4a7b528b81f1)
(cherry picked from commit 22157646da56cba346893ea17a1de5e4fd689b53)
libm: optimistic pow calling.

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

Change-Id: I6f3aa734adbf99484b7ff70736ef83a41e5815b8
(cherry picked from commit e7fd29685a75f1d26061ac511e7a29479fd871b5)
(cherry picked from commit 7916899916973ca5695526ae45283e54709df32a)
(cherry picked from commit d39a7f744c78409949482e3cb3c9710e8f281a33)