[Bug target/104853] [RISC-V] -march=rv64g not including extension Zifencei

[i at rvalue dot moe]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104853

--- Comment #1 from rvalue <i at rvalue dot moe> ---
It do works with `-march=rv64g_zifencei`, but I don't think it's the right way
to work.