* common/linux-ptrace.c: Change __i386__ to __i386__ || __x86_64__.
(linux_ptrace_test_ret_to_nx): Extend comment for x86_64. Change
__i386__ to __i386__ || __x86_64__. Extend code also for __x86_64__.
Extend code also for PaX support. Convert all gdb_assert to warning
calls.