Make configure probe for mbstowcs_l as well as wcstombs_l.

We previously supposed that any given platform would supply both or neither
of these functions, so that one configure test would be sufficient.  It now
appears that at least on AIX this is not the case ... which is likely an
AIX bug, but nonetheless we need to cope with it.  So use separate tests.
Per bug #6758; thanks to Andrew Hastie for doing the followup testing
needed to confirm what was happening.

Backpatch to 9.1, where we began using these functions.