It's odd, but because Windows requires a full Python3 install we must
have an executable called "tools/python3/python3" to use Python3 in the
toolchain.

Before, we simply symlinked to /usr/bin/python3 (for Linux) or
/usr/local/bin/python3 (Mac).  Unfortunately, depending on the method of
installation, on MacOS the Python3 executable can be in /usr/bin/python3
instead.

To avoid the entire issue, unify the Mac and Linux python3 placeholders
to use python3 itself to jump to the real executable.

Fixes esp8266#6931

The tar extraction for the updated python3 tarball will fail on systems
that already have a symlink in /tools/python3/python3 because the tar
extractor attempts to open the *target of the symlink* (i.e. the actual
interpreter in /usr/bin or /usr/local/bin).

Add a commented hack to destroy this symlink before expanding the
tarballs, if the file exists.  This is safe to do since it will be
overwritten by any extractions of the python3 tarball later in the
process.