gdb/testsuite: Fix file location for gdb.base/backtrace-through-cu-nodebug

The newly added test gdb.base/backtrace-through-cu-nodebug.exp had a
problem in the call to gdb_compile, that caused the .o files to be
outputted in the GDB file tree. This commit fixes the issues in the calls.

Reported-By: Tom de Vries <tdevries@suse.de>
Approved-By: Tom de Vries <tdevries@suse.de>