bpo-40947: getpath.c uses PyConfig.platlibdir (GH-20807)

Followup of bpo-40854, there is one remaining usage of PLATLIBDIR
which should be replaced by config->platlibdir.

test_sys checks that sys.platlibdir attribute exists and is a string.

Update Makefile: getpath.c and sysmodule.c no longer need PLATLIBDIR
macro, PyConfig.platlibdir member is used instead.

Co-authored-by: Sandro Mani <manisandro@gmail.com>