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.

 # Blackfin specific configuration file

 config ARCH_blackfin

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_DEFAULT_LE

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     select ARCH_SUPPORT_FPU

      help

       The Blackfin architecture