unix/alloc: Add option to use uPy's alloc-exec implementation even for libffi.

When built for Linux, libffi includes very bloated and workaround exec-alloc
implementation required to work around SELinux and other "sekuritee" features
which real people don't use. MicroPython has own alloc-exec implementation,
used to alloc memory for @micropython.native code. With this option enabled,
uPy's implementation will override libffi's. This saves 11K on x86_64 (and
that accounts for more than half of the libffi code size).

TODO: Possibly, we want to refactor this option to allow either use uPy's
implementation even for libffi, or allow to use libffi's implementation even
for uPy.