x86: Refactor kernel-mode NMI handling

This refactor aims to add the ability to share performance counter
interrupts by refactoring the kernel-mode NMI handler. The handler now
allows multiple drivers to service the same interrupt (e.g. hwpmc(4)
and hwt(4)'s Intel Processor Trace backend).

Reviewed by: kib, avg
Differential Revision: https://reviews.freebsd.org/D46421