GCM: move look-up table to .data section and unshare between processes

* cipher/cipher-gcm.c (ATTR_ALIGNED_64): New.
(gcmR): Move to 'gcm_table' structure.
(gcm_table): New structure for look-up table with counters before and
after.
(gcmR): New macro.
(prefetch_table): Handle input with length not multiple of 256.
(do_prefetch_tables): Modify pre- and post-table counters to unshare
look-up table pages between processes.
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GnuPG-bug-id: 4541
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>