esp32,unix: Support building C++ code.

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++.

tests/unix/extra_coverage.py.exp

@@ -144,6 +144,7 @@ OSError
 0
 None
 None
+cpp None
 frzstr1
 frzstr1.py
 frzmpy1