Add ARMv8/AArch64 Crypto Extension implementation of GCM

* cipher/Makefile.am: Add 'cipher-gcm-armv8-aarch64-ce.S'.
* cipher/cipher-gcm-armv8-aarch64-ce.S: New.
* cipher/cipher-internal.h (GCM_USE_ARM_PMULL): Enable on
ARMv8/AArch64.

Benchmark on Cortex-A53 (1152 Mhz):

Before:

|  nanosecs/byte   mebibytes/sec   cycles/byte

GMAC_AES | 15.54 ns/B 61.36 MiB/s 17.91 c/B

After (11.9x faster):

|  nanosecs/byte   mebibytes/sec   cycles/byte

GMAC_AES | 1.30 ns/B 731.5 MiB/s 1.50 c/B

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>