Fix swap-bytes-64 on 32-bit CCL.
Make it faster by using two swap-bytes-32 instead of swapping everything by hand.
Improve SBCL:
On x86-64, for swap-bytes-16 use ROL WORD-REG, 8. XCHG AH, AL could be
faster, but x86-64 SBCL doesn't support AH/BH/CH/DH-registers.
On x86, optimize swap-bytes-64 by implementing it in terms of swap-bytes-32.