Issue #6834: replace the implementation for the 'python' and 'pythonw' executables...

Issue #6834: replace the implementation for the 'python' and 'pythonw' executables on OSX.

The previous implementation used execv(2) to run the real interpreter, which means that
you cannot use the arch(1) tool to select the architecture you want to use for a
universal build because that only affects the python/pythonw wrapper and not the actual
interpreter.

The new version uses posix_spawnv with a number of OSX-specific options that ensure that
the real interpreter is started using the same CPU architecture as the wrapper, and that
means that 'arch -ppc python' now actually works.

I've also changed the way that the wrapper looks for the framework: it is now linked to
the framework rather than hardcoding the framework path. This should make it easier to
provide pythonw support in tools like virtualenv.