Fix the Issue #65: NMI handler live patching

This fixes issue #65, NMI handler live patching support. The basic idea is already discussed on that issue, and this is an implementation of that.

Short description: Live patching on NMI handler must be failed if an NMI happens when checking the backtrace on each task, because it can cause inconsistency between non-patched and patched functions which concurrently run. This adds an atomic counter to detect such concurrently running NMI handler while
updating.