[intrng] Migrate the intrng code from sys/arm/arm to sys/kern/subr_intr.c.

The ci20 port (by kan@) is going to reuse almost all of the intrng code
since the SoC in question looks suspiciously like someone took an ARM
SoC design and replaced the ARM core with a MIPS core.

• migrate out the code;
• rename ARM_ -> INTR_;
• rename arm_ -> intr_;
• move the interrupt flush routine from intr.c / intrng.c into arm/machdep_intr.c - removing the code duplication and removing the ARM specific bits from here.

Thanks to the Star Wars: The Force Awakens premiere line for allowing
me a couple hours of quiet time to finish the universe builds.

Tested:

• make universe

TODO:

• The structure definitions in subr_intr.c still includes machine/intr.h which requires one duplicates all of the intrng definitions in the platform code (which kan has done, and I think we don't have to.)
  
  Instead I should break out the generic things (function declarations, common intr structures, etc) into a separate header.

• Kan has requested I make the PIC based IPI stuff optional.