(Imported from upstream's 25d14c6c29b53907bf614b9964d43cd98401a7fc.)
At the same time remove miniscule bias in final subtraction. Performance
penalty varies from platform to platform, and even with key length. For
rsa2048 sign it was observed to be 4% for Sandy Bridge and 7% on
Broadwell.
(This is part of the fix for CVE-2016-0702.)
Change-Id: I43a13d592c4a589d04c17c33c0ca40c2d7375522
Reviewed-on: https://boringssl-review.googlesource.com/7244
Reviewed-by: Adam Langley <agl@google.com>
b360eaf001