package/gdb: prevent gdbserver being selected for RISC-V builds

There is currently no version of gdbserver for RISC-V. Until this
is implemented we will prevent both the direct and indirect
selection of gdbserver for RISC-V builds. In practice this means
that 'cross gdb for the host' cannot be selected and that
'full debugger' must be automatically selected for the gdb target
package.

[Peter: simplify logic, add comment]
Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>