[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>