Add a few more constant-time utility functions.

Imported from upstream's 9bed73adaa6f834177f29e478d9a2247a6577c04.

Upstream's commit appears to have been based on BoringSSL's commits to
improve the constant-time behaviour of RSA padding checks and thus I've
not tried to import those bits of the change.

Change-Id: I0ea5775b0f1e18741bbbc9f792a6af0d3d2a4caf
2 files changed