busdma: deduplicate _bus_dmamap_addseg() function

It is functionally identical in all implementations, so move the
function to subr_busdma_bounce.c. The KASSERT present in the x86 version
is now enabled for all architectures. It should be universally
applicable.

Reviewed by: jhb
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45047