Revert "C: use cc, not cxx, to link programs."

This reverts commit dccd99c8c8d9b1390f96adb2414c510d3ff85278.

Turns out there's a good reason to link C++ programs using cxx
rather than cc: it's how they pick up implicit dependencies on
libstdc++ and friends.  In the long term this should become
configurable.  For now, we'll just use cxx every time.

Change-Id: I76405c0de338a2bca96f4775a178c78a321dc207