powerpc: Implement fpu_kern_enter/fpu_kern_leave

Summary:
Provide an implementation of fpu_kern_enter/fpu_kern_leave for PPC to
enable FPU, VSX, and Altivec usage in-kernel. The functions currently
only support FPU_KERN_NOCTX, but this is sufficient for ossl(1) and many
other users of the API.

This patchset has been tested on powerpc64le using a modified version of
the in-tree tools/tools/crypto/cryptocheck.c tool to check for FPU/Vec
register clobbering along with a follow-up patch to enable ossl(4) on
powerpc64*.

Reviewed by: jhibbits
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D41540