gdb: query inferior's filesystem for build-id debug files

This fixes a bug related to build-id files with linux namespaces.
Specifically, we expect the debug files to be present inside the container,
thus the container filesystem should be queried if the program is running
inside one.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32956
Approved-By: Andrew Burgess <aburgess@redhat.com>