Add -Z external-sanitizer-runtime

This adds the unstable `-Z external-sanitizer-runtime` flag that will
prevent rustc from emitting linker paths for the in-tree LLVM sanitizer
runtime library.

compiler/rustc_session/src/options.rs

@@ -1645,6 +1645,8 @@ options! {
         "emit the bc module with thin LTO info (default: yes)"),
     export_executable_symbols: bool = (false, parse_bool, [TRACKED],
         "export symbols from executables, as if they were dynamic libraries"),
+    external_clangrt: bool = (false, parse_bool, [UNTRACKED],
+        "rely on user specified linker commands to find clangrt"),
     extra_const_ub_checks: bool = (false, parse_bool, [TRACKED],
         "turns on more checks to detect const UB, which can be slow (default: no)"),
     #[rustc_lint_opt_deny_field_access("use `Session::fewer_names` instead of this field")]