Some phones have a buggy NEON unit and the Poly1305 NEON code fails on
them, even though other NEON code appears to work fine.

This change:

1) Fixes a bug where NEON was assumed even when the code wasn't compiled
   in NEON mode.

2) Adds a second NEON control bit that can be disabled in order to run
   NEON code, but not the Poly1305 NEON code.

https://code.google.com/p/chromium/issues/detail?id=341598

Change-Id: Icb121bf8dba47c7a46c7667f676ff7a4bc973625
Reviewed-on: https://boringssl-review.googlesource.com/1351
Reviewed-by: Adam Langley <agl@google.com>