[clang] [llvm] [RISCV] Add back SiFive's cdiscard.d.l1 and cflush.d.l1 instructions. (PR #83896)

[Jessica Clarke via cfe-commits]

https://github.com/jrtc27 requested changes to this pull request.

These need the vendor "sf." prefix

https://github.com/llvm/llvm-project/pull/83896