freebsd build: always provide a __pmDumpStack in libpcp
freebsd build: always provide a __pmDumpStack in libpcp

If neither backtrace nor trace_back_stack are available,
the build fails with link errors.  Provide a __pmDumpStack
stub for any calls on those platforms.

andyvand/cygpcpfans,prasincs/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,mbaldessari/pcp,aeg-aeg/pcpfans,adfernandes/pcp,edwardt/pcp,edwardt/pcp,prasincs/pcp,adfernandes/pcp,edwardt/pcp,adfernandes/pcp,tjanez/pcp,edwardt/pcp,mbaldessari/pcp,wuliming/pcp,tjanez/pcp,aeg-aeg/pcpfans,tjanez/pcp,wuliming/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,andyvand/cygpcpfans,mbaldessari/pcp,edwardt/pcp,wuliming/pcp,prasincs/pcp,tjanez/pcp,tjanez/pcp,wuliming/pcp,aeg-aeg/pcpfans,tjanez/pcp,tjanez/pcp,adfernandes/pcp,adfernandes/pcp,prasincs/pcp,wuliming/pcp,aeg-aeg/pcpfans,prasincs/pcp,andyvand/cygpcpfans,wuliming/pcp,edwardt/pcp,adfernandes/pcp,prasincs/pcp,prasincs/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,edwardt/pcp,mbaldessari/pcp,andyvand/cygpcpfans,edwardt/pcp,andyvand/cygpcpfans,mbaldessari/pcp,adfernandes/pcp,mbaldessari/pcp,wuliming/pcp,adfernandes/pcp,aeg-aeg/pcpfans,wuliming/pcp,tjanez/pcp,prasincs/pcp