gdb: gdbserver package dependency fix (#11179)

In the current trunk, gdbserver is not building anymore (with EGLIBC 2.15
selected) due to a missing dependency:
Package gdbserver is missing dependencies for the following libraries:
libthread_db.so.1

In order to fix it, the system shared lib libthread_db.so.1 is copied from
the toolchain lib dir into the gdb package target structure (/usr/lib) to
pass the dependency checks.
This patch was (quickly) reviewed by Paul Fertser on IRC.

Signed-off-by: xxiao <xxiao@fosiao.com>
Signed-off-by: Matthieu Boujonnier <openwrt-devel@metaverse.fr>

SVN-Revision: 33479