Fix SHA-384 and -512 in NO_ASM mode.

This transcription bug comes from the start of BoringSSL and, as you can
imagine, was a complete delight to track down.

Change-Id: I3051934195098a1d3bf893b154389ec7f14d3609
Reviewed-on: https://boringssl-review.googlesource.com/3740
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>