* gcm.c (gcm_gf_mul_chunk): Special case first and last iteration.

(gcm_gf_add): New function, a special case of memxor. Use it for
all memxor calls with word-aligned 16 byte blocks. Improves
performance to 152 cycles/byte with no tables, 28 cycles per byte
with 4-bit tables and 10.5 cycles per byte with 8-bit tables.

Rev: nettle/gcm.c:1.7