Bug 1671424 - Move configure execution from client.mk to `mach configure`. r=firefox-build-system-reviewers,rstewart

`mach configure` currently runs the equivalent to `make -f client.mk`.
This is history, and essentially does the following:
- Create `configure` and `js/src/configure` from `configure.in` and
`js/src/configure.in` respectively.
- Create the objdir.
- Run `configure` from the objdir.

The `configure` script is, nowadays, only really used as a means to set
OLD_CONFIGURE (and also for people who want to run `configure`,
literally, as in the `configure; make` workflow). `mach configure`
actually doesn't need it. Neither does recursing into `js/src` require
`js/src/configure`, since bug 1520340 (and now as of bug 1669633, we
don't even recurse).

Because configure.py can actually derive OLD_CONFIGURE on its own
(except for `js/src/configure`, but `mach configure` doesn't run that),
we don't really need `configure` for `mach configure`.

So all in all, we're at a point in history where it's straightforward to
just initiate configure.py from mach configure, so we just do that.

And in the hypothetical case where the `mach configure` code is somehow
running in python2, we get the mach virtualenv python3 and use it to
execute `configure.py`.

Differential Revision: https://phabricator.services.mozilla.com/D93741