x86: Use sx lock for interrupt sources.

- Certain pic_assign_cpu, e.g. msi_assign_cpu can have quite a long
  call chain.  For msi_assign_cpu, mutex makes complex PCI bridge
  drivers more tricky, e.g. sleep can note be called, etc, it will
  be pretty tricky for upcoming Hyper-V PCI bridge driver for PCI
  pass-through.
- It is not used on any hot code path nor non-sleepable context, so
  sx should have the same effect as mutex.

PIC list is still protected by mutex to keep suspend/resume work.

Discussed with: jhb
Reviewed by:    jhb
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7784