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glibc, toolchain-external: copy libthread_db when gdb is enabled

As Samuel Martin noticed, libthread_db is not only needed when
cross-gdb+gdbserver is used, but also when the native gdb is used on
the target. As a consequence, this patch modifies the glibc package
and the external toolchain logic to ensure that libthread_db is copied
to the target either when the native gdb or gdbserver is enabled, by
relying on the BR2_PACKAGE_GDB option, which is enabled when native
gdb and/or gdbserver are enabled.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>