Provide npx_get_fsave(9) and npx_set_fsave(9) functions to obtain and
restore the FPU state from the format of machine FSAVE area. The
intended use is for ABI emulators to provide FSAVE-formatted FPU state
to usermode requiring it, while kernel could use FXSAVE due to
XMM/XSAVE.

The core functionality to convert from/to FXSAVE format is shared with
the fill_fpregs_xmm() and set_fpregs_xmm(). Move the later functions
to npx.c and rename them to npx_fill_fpregs_xmm() and
npx_set_fpregs_xmm(). They differ from nptx_get/set_fsave(9) since
our mcontext contains padding to be zeroed or ignored.

fill_fpregs() and set_fpregs() could be converted to use the new
interface, but there are small differences to handle.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week