Support building .cpp files and linking them into the micropython
executable in a way similar to how it is done for .c files. The main
incentive here is to enable user C modules to use C++ files (which are put
in SRC_MOD_CXX by py.mk) since the core itself does not utilize C++.
However, to verify build functionality a unix overage test is added. The
esp32 port already has CXXFLAGS so just add the user modules' flags to it.
For the unix port use a copy of the CFLAGS but strip the ones which are not
usable for C++.
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