libc/glibc: repair debugging with gdb (glibc 2.9)

Without this patch, crosstool-ng-built glibc-2.9 prevents
debugging any exeutable with gdb.
gdb says:
[Thread debugging using libthread_db enabled]
find_new_threads_callback: cannot get thread info: generic error
See also https://bugzilla.redhat.com/show_bug.cgi?id=487212
for a discussion of the bug and the solution.

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =