toolchain: do not build GDB when using an external toolchain

The external toolchain should provide it if needed.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36802 3c298f89-4303-0410-b956-a3cf2f4a3e73