Fix linking with Python 3.8

Fix linking with Python 3.8

The behaviour of python-config --libs has changed in Python 3.8.

For example, compare the output of it with Python 3.7 and 3.8 on an
ArchLinux system:

    $ python3.7-config --libs
    -lpython3.7m -lcrypt -lpthread -ldl  -lutil -lm
    $ python3.8-config --libs
    -lcrypt -lpthread -ldl  -lutil -lm -lm
    $

The lack of -lpython in the latter case causes the linker to fail when
attempting to build Yosys against Python 3.8.

Passing the new --embed flag to python-config adds -lpython, just like
earlier versions of Python:

    $ python3.8-config --embed --libs
    -lpython3.8 -lcrypt -lpthread -ldl  -lutil -lm -lm
    $

This commit adds code for automatically detecting support for the
--embed flag. If it is supported, it is passed to all python-config
invocations. This fixes building against Python 3.8.