SERVER-7080 Link to backtrace, backtrace_symbols and backtrace_symbols_fd at runtime

Do not make direct calls to backtrace, backtrace_symbols or backtrace_symbols_fd,
which are present in Solaris 11 but not in Solaris 10.  Instead, see if they are
available in a loaded library (which will be libc.so.1) at runtime and either
call them or call an emulation.