graphics/libjxl: enable AVX512 runtime detection (amd64-only)

Upstream default is optimized for bundling or Linux-style micro-packages.
FreeBSD currently doesn't split docs/headers/plugins, so minor binary size
increase (7 MiB -> 9 MiB) is less important. FreeBSD is primarily used on
servers where AVX512 is more common while on old desktops libjxl bloat is
dwarfed by many other packages (e.g., firefox, llvm*, gcc*, boost-libs).

See also https://github.com/libjxl/libjxl/pull/2627#issuecomment-1618237136