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

[Ana Pazos via cfe-commits]

apazos wrote:

Also, @topperc, we should add the new extensions  to the list of vendor extensions in the specs:
ttps://github.com/riscv-non-isa/riscv-toolchain-conventions/tree/master?tab=readme-ov-file#list-of-vendor-extensions

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