contain index values shifted by the size of a word, and with 2
added. This saves some additions in the sparc assembler code.
Updates aes-encrypt-table.c and aes-decrypt-table.c.

* aes-internal.h (struct aes_table): Renamed the shift_idx field
to sparc_idx, as it will be tweaked to improve the sparc code.
Also reduced its size to [2][4].
(IDX_FACTOR): Deleted constant.
* aes-encrypt-table.c (_aes_encrypt_table): Adapted initializer of
sparc_idx.
* aes-decrypt-table.c (_aes_decrypt_table): Likewise.
* asm.m4: Deleted AES_SIDX2, to match struct aes_table.

Rev: src/nettle/aes-decrypt-table.c:1.3
Rev: src/nettle/aes-encrypt-table.c:1.3
Rev: src/nettle/aes-internal.h:1.8