[clang] [RISCV] Implement shadow stack on shadow stack mode with Zicfiss. (PR #68075)

[Sami Tolvanen via cfe-commits]

samitolvanen wrote:

The patch basically changes the ShadowCallStack back-end to emit an sspush/sspopchk instead of the usual SCS push/pop, which seems like a reasonable approach to me. However, it would be helpful to mention the dependency on `-fsanitize=shadow-call-stack` in the commit message, and you should also update [the documentation](https://clang.llvm.org/docs/ShadowCallStack.html).

https://github.com/llvm/llvm-project/pull/68075