libcxxrt: don't export nonexistant symbols

Remove version entries that we don't build.

Add an arm specific Version.map and for other targets run the files
through sed to handle int vs long in new and delete.

Ideally we'd use the SYMBOL_MAPS functionality to preprocess with cpp,
but it doesn't currently handle C++ symbols so be annoyingly duplicative
for now.

Differential Revision: https://reviews.freebsd.org/D44325