hmac: Add support for SHA-512 in GHmac

The block size wasn’t configured before, so calling g_hmac_new() with
G_CHECKSUM_SHA512 would hit a g_assert_not_reached() and explode.

Implement G_CHECKSUM_SHA512 and add unit tests for HMACs with SHA-256
and SHA-512 using the test vectors from RFC 4868.

https://bugzilla.gnome.org/show_bug.cgi?id=724741