Expose doreti as a global symbol on amd64 and i386.

doreti provides the common code path for returning from interrupt
andlers on x86.  Exposing doreti as a global symbol allows kernel
modules to include low-level interrupt handlers instead of requiring
all low-level handlers to be statically compiled into the kernel.

Submitted by:       Howard Su <howard0su@gmail.com>
Reviewed by:        kib

hyperv: Deprecate HYPERV option by moving Hyper-V IDT vector into vmbus

Submitted by:       Jun Su <junsu microsoft com>
Reviewed by:        jhb, kib, sephe
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5910