package/gdb: prevent gdbserver being selected for or1k builds

There is currently no version of gdbserver for or1k. Until this
is implemented we will prevent both the direct and indirect
selection of gdbserver for or1k 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.

This partially reverts commit 991b7b99
which claimed that gdbserver for or1k was already supported before
version 8.3. That is not true - the commit that adds gdbserver support
for or1k [1] was only merged for version 12.1, which hasn't been
integrated in Buildroot yet.

Without that support, the build of gdbserver fails with

/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/or1k-buildroot-linux-musl/11.2.0/../../../../or1k-buildroot-linux-musl/bin/ld: server.o: in function `main':
server.cc:(.text.startup+0x6dc): undefined reference to `initialize_low()'
/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/or1k-buildroot-linux-musl/11.2.0/../../../../or1k-buildroot-linux-musl/bin/ld: remote-utils.o: in function `prepare_resume_reply(char*, ptid_t, target_waitstatus*)':
remote-utils.cc:(.text+0x28a8): undefined reference to `using_threads'
/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/or1k-buildroot-linux-musl/11.2.0/../../../../or1k-buildroot-linux-musl/bin/ld: remote-utils.cc:(.text+0x28b0): undefined reference to `using_threads'

Fixes: http://autobuild.buildroot.net/results/b3c/b3c0df53d09d9facaf0c3c2bc4529f9fcf7737ee

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=4933265c3f71b9134363d0c05f09542d5cc677f4



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Stafford Horne <shorne@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>