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

[schwab--- via Gcc-bugs]

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

--- Comment #10 from Andreas Schwab <sch...@linux-m68k.org> ---
Or "other ABI-mandated fixed roles".  This also includes return value
registers.