Commit 97e23896a9168132b6d36ca22ae1af10dd53d80d regressed this behavior
because it made target_util.supportsStackProtector *correctly*
notice which zig backend is being used to generate code, while the
logic calling that function *incorrectly assumed* that .zig code is being
compiled, when in reality it might be only C code being compiled.

This commit adjusts the option resolution logic for stack protector so
that it takes into account the zig backend only if there is a zig
compilation unit. A separate piece of logic checks whether clang
supports stack protector for a given target.

closes #18009
closes #18114
closes #18254