[gdb/testsuite] Handle ptrace operation not permitted in can_spawn_for_attach When running the testsuite on a system with kernel.yama.ptrace_scope set to 1, we run into attach failures. Fix this by recognizing "ptrace: Operation not permitted" in can_spawn_for_attach. Tested on aarch64-linux and x86_64-linux. Approved-By: Pedro Alves <pedro@palves.net>