[Bug target/109456] `-ffixed` is ignored for `a` registers on RISC-V.

[gccriscvuser at proton dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109456

--- Comment #6 from gccriscvuser at proton dot me ---
I'm willing to agree that the generator is emitting correct code, but in that
case, there should be an error- or fatal-level diagnostic indicating that
`-ffixed` is not supported for `a4` (solution 3 above).

It would be even better if the diagnostic were issued only if there is at least
one function that takes so many parameters that `a4` is needed.  The diagnostic
message could point at the function(s) themselves to assist the programmer in
correcting the issue.  If none of the functions use `a4` as a parameter, then
no diagnostic is needed.