Add domain policy allocation for amd64 fpu_kern_ctx

Like other types of allocation, fpu_kern_ctx are frequently allocated per-cpu.
Provide the API and sketch some example consumers.

fpu_kern_alloc_ctx_domain() preferentially allocates memory from the
provided domain, and falls back to other domains if that one is empty
(DOMAINSET_PREF(domain) policy).

Maybe it makes more sense to just shove one of these in the DPCPU area
sooner or later -- left for future work.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D22053