Refactor GCM C implementation.

Refactor GCM C implementation.

Assembly implementations not yet updated.

* Makefile.in (nettle_SOURCES): Add ghash-set-key.c ghash-update.c.
(DISTFILES): Replaced gcm-internal.h with ghash-internal.h.

* testsuite/gcm-test.c (test_ghash_internal): Updated to use
_ghash_set_key and _ghash_update.

* gcm.c (INC32): Deleted macro, used in only one place.
(gcm_set_key): Update to use _ghash_set_key.
(gcm_hash): Renamed, was _gcm_hash, and implemented in terms of
_ghash_update.
(bswap_if_le): New function (copied from nist-keywrap.c).
(gcm_hash_sizes): Use bswap_if_le and _ghash_update.
(gcm_set_iv): Updated to use gcm_hash and block16_zero.
(gcm_digest): Use _ghash_digest.

* ghash-internal.h: New file, declaring new internal ghash interface.
* gcm-internal.h: Deleted file.
* ghash-update.c (gcm_gf_shift_8): Moved here (from gcm.c)
(gcm_gf_mul): Likewise.
(_ghash_update): New function, extracted from _nettle_gcm_hash_c.
(_ghash_digest): New function.
* ghash-set-key.c (_ghash_set_key): New file and function.
Extracted from _nettle_gcm_init_key_c and _nettle_gcm_set_key.