This change adds a function so that an ECDH and the hashing of the
resulting 'x' coordinate can occur inside the FIPS boundary.

Change-Id: If93c20a70dc9dcbca49056f10915d3ce064f641f
Reviewed-on: https://boringssl-review.googlesource.com/30104
Reviewed-by: Adam Langley <agl@google.com>