Rename hexdump to hexdump_write.

It's a static function anyway so it doesn't affect anything and it's
colliding with a debugging function on one platform.

Change-Id: Iae0595cce7cb2bdd4c56217f6f1de51ff3134a8b
Reviewed-on: https://boringssl-review.googlesource.com/2630
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>