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
MFC after: 3 weeks
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D7784