gdb: do not allow gdbserver/cross-gdb build in some cases

When an external toolchain is used, and the user has chosen to copy
the external toolchain gdbserver to the target, then we should allow
the user to build a gdbserver and/or a cross-gdb: the ones of the
external toolchain should be used.

The reasoning is that one must use a gdbserver and cross-gdb of
identical versions to be sure that debugging will work properly.

Change suggested by Yann E. Morin.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>