g10/armor: use libgcrypt's CRC24 implementation

* g10/armor.c (CRCINIT, CRCPOLY, CRCUPDATE, crc_table): Remove.
(new_armor_context): Open libgcrypt CRC24 context.
(release_armor_context): Close CRC24 context.
(initialize): Remove CRC table generation.
(get_afx_crc): New.
(check_input, fake_packet, radix64_read, armor_filter): Update to use
CRC24 context.
* g10/filter.h (armor_filter_context_t): Replace crc intermediate value
with libgcrypt md context pointer.

This patch changes armor filter to use optimized CRC24 implementation
from libgcrypt to speed up encryption and decryption.

Benchmark results below, tested on Intel Core i7-4790K (turbo off).
Encrypted 2 GiB through pipe to ramfs file using AES128. Decrypt
ramfs file out through pipe to /dev/null.

before patch-set

gpg process

armor: user time pipe transfer rate
encrypt-aead: 13.8 140 MB/s
decrypt-aead: 30.6 68 MB/s
encrypt-cfb: 17.4 114 MB/s
decrypt-cfb: 32.6 64 MB/s

after (decrypt+iobuf+crc opt)

gpg process

armor: user time pipe transfer rate
encrypt-aead: 8.7 211 MB/s
decrypt-aead: 17.6 116 MB/s
encrypt-cfb: 12.6 153 MB/s
decrypt-cfb: 19.6 105 MB/s

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