Timing fix mitigation for FIPS mode.

Timing fix mitigation for FIPS mode.
We have to use EVP in FIPS mode so we can only partially mitigate
timing differences.

Make an extra call to EVP_DigestSignUpdate to hash additonal blocks
to cover any timing differences caused by removal of padding.
(cherry picked from commit b908e88ec15aa0a74805e3f2236fc4f83f2789c2)