* x86/aes-encrypt-internal.asm: Updated use of AES_SUBST_BYTE.

* x86/aes-decrypt-internal.asm: Likewise.
* x86/aes.m4 (BYTEREG): New macro.
(AES_SUBST_BYTE): Take state registers as argument. Use BYTEREG to
get the corresponding byte register.

Rev: nettle/x86/aes-decrypt-internal.asm:1.2
Rev: nettle/x86/aes-encrypt-internal.asm:1.2
Rev: nettle/x86/aes.m4:1.2