Improve FPU Tag Word reconstruction on i386 to indicate register states.

Improve the code reconstructing en_tw in struct fpreg32 from FXSAVE
results so that all register states are indicated correctly. The
previous code unconditionally mapped non-empty register state to
'normalized value' constant. The new code explicitly distinguishes
the 'zero value' and 'special value' constants as well. This improves
consistency between real FSAVE and translation from FXSAVE, and
ensures that tests using PT_GETFPREGS can rely on a single correct
value independently of the underlying implementation.

PR: 250454
Sponsored by: The FreeBSD Foundation
Obtained from: Moritz Systems
Submitted by: Michał Górny <mgorny@moritz.systems>
Discussed with: emaste
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D26856