This is a cherry pick of the loader.py in cppyy-backend
version clingwrapper-1.7.2, which has the new treatment of
CLING_STANDARD_PCH and its setting to 'none' to prevent
the PCH check by Cppyy.

A complete update will come for the three packages of Cppyy
once the functionality of this PR:
https://github.com/root-project/root/pull/3579
is merged into TCling.