const-time: add 64-bit fast paths for const-time buffer functions

* src/const-time.c (_gcry_ct_not_memequal, _gcry_ct_memequal): Add
64-bit processing for larger than 7 byte inputs.

Cherry-pick master commit of:
11e8f4a8de263d9199479b290b8550e6a6ecc619

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>