xz: 5.4.6

[hovercats]

bumps xz to 5.4.6.

edit: xz came out with another release after making this pr, so I bumped it

[michaelforney]

Applied with a few changes:

• I added a probe for __builtin_bswap16 so that it wouldn't try to use it when building with cproc.
• Similarly, I guarded HAVE_USABLE_CLMUL behind a probe for _mm_clmulepi64_si128.
• crc64_small.c should only be built with HAVE_SMALL, so shouldn't be added to the source list. I checked and HAVE_SMALL doesn't actually save that much, and since xz is already kind of slow, I think it's better to go with the faster code even if it is a bit bigger.