npxprobe1(), set npxdna_func to new npxdna_empty(). This function just
returns 0 and thus fallbacks to a T_DNA trap (and math emulate, if present).
This fixes kern/15828 by Masaru Oki, tested on 386DX machine.
Reviewed by Jason Thorpe.
make npxdna_s87() and npxdna_xmm() static
g/c (npx_type == NPX_NONE) test from npxdna_s87(), it's no longer needed