arm: Add fpu_kern_alloc_ctx()

This enables the use of some out-of-tree crypto libraries on arm.
No functional change intended, there are no callers of this function in
the tree currently.

Reviewed by: andrew
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D42969