xen/intr: move interrupt allocation/release to architecture

Simply moving the interrupt allocation and release functions into files
which belong to the architecture. Since x86 interrupt handling is quite
distinct from other architectures, this is a crucial necessary step.

Identifying the border between x86 and architecture-independent is
actually quite tricky. Similarly, getting the prototypes for the
border right is also quite tricky.

Inspired by the work of Julien Grall <julien@xen.org>,
2015-10-20 09:14:56, but heavily adjusted.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D30936