[Bug target/109349] riscv: Add --print-supported-extensions

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109349

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Also from reading the merge request, this really should just be in the gcc user
manual instead of just in the compiler.

Gcc's user manual should document what is supported.
It is how all other targets handle this.