[llvm-branch-commits] [RISCV] Support __builtin_cpu_is (PR #116231)

[Pengcheng Wang via llvm-branch-commits]

https://github.com/wangpc-pp created https://github.com/llvm/llvm-project/pull/116231

We have defined `__riscv_cpu_model` variable in #101449. It contains
`mvendorid`, `marchid` and `mimpid` fields which are read via system
call `sys_riscv_hwprobe`.

We can support `__builtin_cpu_is` via comparing values in compiler's
CPU definitions and `__riscv_cpu_model`.

This depends on #116202.