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

[Ana Pazos via cfe-commits]

apazos wrote:

@topperc , how about the CEASE instruction? Can you add it in this patch?

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