toolchain-external: don't create gdb symlink when building host-gdb

The external-toolchain infrastructure creates symbolic links for all
tools in the host directory. However, when buildroot builds its own
version of a cross debugger (BR2_PACKAGE_HOST_GDB), and the toolchain
also provides a cross debugger, there would be two symbolic links for
gdb in the host directory, which is confusing.
An example use case is where the external toolchain only provides a
64-bit gdbserver (e.g. Cavium Networks SDK) but the target is completely
32-bit (e.g. n32 ABI). In this case, using gdbserver on target requires
copying a bunch of 64-bit libraries to the target as well, just for gdb.
In this case, one can let buildroot build both gdbserver as cross-gdb
(both in 32-bit).

This patch modifies the symlink creation so that no gdb (or gdbtui)
symlink is created if buildroot is going to build a cross-gdb.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>