Some fixes for m68k users (incl/ for FPU emulation)

[Romain Dolbeau <romain%dolbeau.org@localhost>]

(relevant to all m68k, but mac68k is the most likely users of FPU
emulation hence the cross-post; I didn't want to spam all the 68k
lists)

Hello,

In the process of creating my litex68k port to my homegrown
as-for-now-FPGA-based system (version with real CPUs still TBD :-) ),
systematic testing has found some issues in m68k, in particular in the
FPU emulation code.

Those patches are not eligible for submission to "gnats" as they were
identified and fixed mostly by using a LLM, but they might be of
interest to end-users building their own kernel. Hopefully this
notification message (which is written without LLM assistance!) will
be acceptable to the lists. Also, the CPU they were actually tested on
are my own soft-core recreation of the MC680x0 and if anyone ever
tests them on actual silicon, feedback is very welcome! (all my '040
machines have full MC68040).

The list of patches and brief description can be found here:
<https://github.com/MelkhiorVintageComputing/netbsd-litex-m68k/blob/litex68k/UPSTREAM.md>
(beware: LLM-generated). The most relevant are the FPU emulation
patches, there's also a couple minor performance improvements that
could be relevant. The SMP patches are probably of interest to no-one
but me as other than my litex68k port, there's no SMP support for
m68k.

Cordially,

-- 
Romain Dolbeau