Semihosting, arm, riscv, ppc and common code

[Benjamin Herrenschmidt]

Hi Folks !

So I started "porting" over (read: copying) the arm semihosting code to
ppc to mimmic what Keith did for risv (mostly for picolibc support).

I noticed that the bulk of arm-semi.c (or riscv-semi.c) is trivially
made completely generic by doing a couple of changes:

 - Make most functions take a CPUState instead of the architecture
specific "env"

 - Provide arch helpers to retreive the op, set the result, do the
flen bug hack and possibly a couple of others (I'm not done yet).

There are other archs who do semihosting completely differently but at
least those 3 can share code.

Any objection/comment on the approach ? What I'll probably do is get
things going first with my ppc version (which I made more/less generic
but still located in target/ppc) at which point I can post an RFC so
you get an idea, and we find a good location for it.

>From thre we might consider fixing some of the worst crap in there
doing backwards compatible extensions if we care enough :-)

Cheers,
Ben.