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++.

ports/unix/coveragecpp.cpp

@@ -0,0 +1,23 @@
+extern "C" {
+#include "py/obj.h"
+}
+
+#if defined(MICROPY_UNIX_COVERAGE)
+
+// Just to test building of C++ code.
+STATIC mp_obj_t extra_cpp_coverage_impl() {
+    return mp_const_none;
+}
+
+extern "C" {
+mp_obj_t extra_cpp_coverage(void);
+mp_obj_t extra_cpp_coverage(void) {
+    return extra_cpp_coverage_impl();
+}
+
+// This is extern to avoid name mangling.
+extern const mp_obj_fun_builtin_fixed_t extra_cpp_coverage_obj = {{&mp_type_fun_builtin_0}, {extra_cpp_coverage}};
+
+}
+
+#endif