[llvm] [RISCV] Get host CPU name via hwprobe (PR #142745)

[Jessica Clarke via llvm-commits]

================
@@ -1672,8 +1672,29 @@ StringRef sys::getHostCPUName() {
   return "generic";
 }
 #elif defined(__riscv)
+// struct riscv_hwprobe
+struct RISCVHwProbe {
----------------
jrtc27 wrote:

This is Linux-specific, it should be guarded as such

https://github.com/llvm/llvm-project/pull/142745