[llvm] [Offload] Add support for riscv64 to host plugin (PR #115773)

[Shilei Tian via llvm-commits]

shiltian wrote:

`libomp` doesn't support RISC-V yet. There is a PR but the author doesn't know whether it works properly or not.

https://github.com/llvm/llvm-project/pull/115773