drsyms doesn't find libc symbols on some *nix

On some *nix builds, the libc debug files do not have the `.debug` extension, making it impossible to find them on the conventional search path (as specified in the [gdb docs](http://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html)). This may cause some tests to fail, e.g. `wrap_malloc` in Dr. Memory, which sometimes needs libc symbols to identify common suppressions.