Fix flag_if_supported when OUT_DIR not set

[NobodyXu]

Reference rust-lang/rust#158169

Use a tempfile if not exists

[cestercian]

I'll take this. is_flag_supported_inner / ensure_check_file currently require OUT_DIR (or Build::out_dir()), so the probe errors and flag_if_supported does .unwrap_or(false) and drops the flag. Plan is to keep get_out_dir as-is for compile, and fall back to a NamedTempfile under std::env::temp_dir() only for the flag-support probe when OUT_DIR is unset. Add a test that does not set out_dir / OUT_DIR and still accepts a universally supported flag.