This commit enables stage2 end-to-end tests to run natively on macOS
(where and when applicable). Since QEMU on macOS doesn't support
the same type of architecture emulation as it does on linux (i.e.,
there is no `qemu-x86_64` for instance), this commit ensures that we
specify a path to dynamic linker on macOS (`/usr/lib/dyld`) which
is then checked for existence in `std.CrossTarget.getExternalExecutor()`
function, and if exists, we can run the test natively.

Signed-off-by: Jakub Konka <kubkon@jakubkonka.com>