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 <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28687
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5