arch.7: Add a section describing the user address space

Provide some details about the user address space on each platform.
Document platforms which support multiple address spaces. I didn't
exhaustively document each platform's ABIs (e.g., the 32-bit compat ABI
on amd64), but maybe that's worthwhile.

Reviewed by: ziaee, kib, emaste
Differential Revision: https://reviews.freebsd.org/D49931