[clang] Don't assume location of compiler-rt for OpenBSD (llvm#92183)

If the `/usr/lib/...` path where compiler-rt is conventionally installed
on OpenBSD does not exist, fall back to the regular logic to find it.

This is a minimal change to allow OpenBSD cross compilation from a
toolchain that doesn't adopt all of OpenBSD's monorepo's conventions.

Author: Ericson2314

clang/lib/Driver/ToolChains/OpenBSD.cpp

@@ -375,7 +375,8 @@ std::string OpenBSD::getCompilerRT(const ArgList &Args, StringRef Component,
   if (Component == "builtins") {
     SmallString<128> Path(getDriver().SysRoot);
     llvm::sys::path::append(Path, "/usr/lib/libcompiler_rt.a");
-    return std::string(Path);
+    if (getVFS().exists(Path))
+      return std::string(Path);
   }
   SmallString<128> P(getDriver().ResourceDir);
   std::string CRTBasename =