Switch OpenSSLMac from EVP_PKEY_HMAC to HMAC_CTX.

EVP_PKEY_HMAC is just a wrapper over HMAC_CTX, so this is slightly more
efficient. This is also the last consumer of BoringSSL's EVP_PKEY_HMAC,
so the API may be removed after this.

Change-Id: I545914b429b23631efd3cacaa22c6d2e7d165fab
5 files changed