Add a flag which traces which interrupts are being handled by the
StackGuard's HandleInterrupts function.

Change-Id: I18fcf58b03cf6c255d7457d8a55b0a0dadb00931
Reviewed-on: https://chromium-review.googlesource.com/649530
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47828}