gdb: disallow GDB_HOST on external toolchain builds

The cross-gdb is supposed to be part of the external toolchain, so
Buildroot does not need to build it. Moreover, GDB_HOST build
currently fail with:

ln -snf ../../bin/arm-unknown-linux-gnueabi-gdb \
                /home/test/outputs/test-48/staging/usr/arm-unknown-linux-gnueabi/bin/gdb
ln: creating symbolic link `/home/test/outputs/test-48/staging/usr/arm-unknown-linux-gnueabi/bin/gdb': No such file or directory

And even worse: they overwrite the cross-gdb of the external
toolchain!

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