Signed-off-by: André Hentschel <nerv@dawncrow.de>

dbghelp: Provide a default value for DYLD_FALLBACK_LIBRARY_PATH, which is closer to how dyld behaves.

dbghelp: On Mac, get the wineloader path from the target or our own process before resorting to guessing.

This makes backtraces more reliably complete when WINELOADER isn't set
and the loader isn't in a typical location.