Revert "linux-arm: disable tree vectorization when neon fpu is used"

This reverts commit 22fd345df447244bc796aba49c3a0803883c128f.

The gains are not consistent or widespread from this change, and only
benefit a very limited code target
linux-arm: utilize dynamic vectorizer cost model

Per the r202980 backport from gcc trunk, we want to utilize
dynamic vect cost model due to utilizing FDO. This is the default
model on optimization level 3, the default for 2 is cheap, and if
-fvect-cost-model is specified without an arg, then default is used.

Tested via analyzing three profiles for a select few portions of libc_bionic

Change-Id: I840564483621b7561f7d7d7b7806587e6cd9a889