3785 Implement MAP_32BIT flag to mmap() [kernel-only]

This pulls the kernel-side changes from commit dbfb896a so that
applications built on systems supporting MAP_32BIT will run.
User-space exposure (via the MAP_32BIT #define, man pages updates,
and truss output support) were excluded intentionally so that
applications built here do not accidentally enable support for
this "future" feature.
3985 want mdb api function for iterating object symbols
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Adam Leventhal <ahl@delphix.com
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@nexenta.com>