intr/x86: add ioapic_drv_t to reduce number of casts in IO-APIC implementation

void * is handy when you truly do not care about the type. Yet there is
so much casting back and forth in the IO-APIC code as to be hazardous.
Achieve better static checking by the compiler using a typedef.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1457