[RISCV] Add back SiFive's cdiscard.d.l1, cflush.d.l1, and cease instructions. (#83896)

These were in LLVM 17 but removed from LLVM 18 due to an incorrect
extension name being used.

This restores them with new extension names that match SiFive's
downstream compiler. The extension name has been used internally for
some time. It uses XSiFive instead of XSf like the newer extensions.
`cease` did not have an internal extension name so its using the `XSf`
convention.

The spec for the instructions is here
https://sifive.cdn.prismic.io/sifive/767804da-53b2-4893-97d5-b7c030ae0a94_s76mc_core_complex_manual_21G3.pdf
though the extension name is not listed.

Column width in the extension printing had to be changed to accommodate
a longer extension name.