We already handle -X gpython.* starting from a6b993c8 (gpython: Add way
to run it with threads runtime). However any other non-gpython -X option
was leading to failure - for example:

    (z-dev) kirr@deca:~/src/tools/go/pygolang$ gpython -X faulthandler
    unknown option: '-X'

(well the error message was also not good)

However on py3 there are useful -X options that might be handy to use,
for example `-X faulthandler` and `-X importtime`.

-> Add support to pymain to handle those via reexecuting underlying
   interpreter like we already do for -O and -E.