this allows a compiler derivation to provide a
hardeningUnsupportedFlagsByTargetPlatform passthru attr
that will be called with the targetPlatform to determine
the unsupported hardening flags for that platform.
we can do this because even though a clang compiler is
multi-target by nature, cc-wrapper effectively fixes the
target platform at wrapping time. otherwise we'd have to
sniff the intended target at runtime, which wouldn't
be fun at all.
the advantage of using a new attribute instead of
allowing hardeningUnsupportedFlags to optionally be a
function is that hardeningUnsupportedFlags retains its
simple overriding pattern for simple cases (i.e.
`(prev.hardeningUnsupportedFlags or []) ++ [ "foo" ]`
) which will continue to work as long as the bottom-most
function of hardeningUnsupportedFlagsByTargetPlatform
falls back to hardeningUnsupportedFlags.
