scons: Update python-config flags for python3.8

scons: Update python-config flags for python3.8

Starting in python 3.8 the python3-config utility requires the --embed
flag to output -lpython3.8. Without this flag, gem5 won't link to the
python library.

More details: https://bugs.python.org/issue36721
https://github.com/python/cpython/pull/13500

Change-Id: Id9c63577dcd2defa7ae62cc32e042c4a245e7082
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28687
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>