scons: added support of default Python installation on MacOS

scons: added support of default Python installation on MacOS

Recent MacOS versions are distributed with python 2.7.
This version of python is sufficient to compile and run gem5.

This patch allows to use the default python instead of the version provided
by third-party tools (e.g., brew)

The default MacOS LLDB debugger is linked against the default python
installation, which conflicts with Python framework provided by third-party
package systems.

This patch removes the need of gem5 to have multiple python installations
on MacOS, if not explicitly installed.

Change-Id: I98f24804149cb2e04ca432c66d2f57e0296af7b2
Reviewed-on: https://gem5-review.googlesource.com/c/15475
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>