Improvements in i386

[Maxime Villard <max%m00nbsd.net@localhost>]

Now i386 benefits from the same improvements as amd64. There are two
differences though:
 - on non-PAE i386, NOX does not exist. Therefore the mappings all have an
   additional X permission. To benefit from X-less mappings, your CPU must
   support PAE, and your kernel must be GENERIC_PAE.
 - the segments are not large-page-aligned, which means that probably some
   parts of the segments are still mapped with normal pages. It is still more
   optimized than it used to be, but not as much as amd64 is.
The asm code is similar now, and each change made in amd64 can easily be made
in i386 as well.

The commits are here, if you spot any mistakes.

[1] http://mail-index.netbsd.org/source-changes/2016/05/14/msg074672.html
[2] http://mail-index.netbsd.org/source-changes/2016/05/14/msg074673.html
[3] http://mail-index.netbsd.org/source-changes/2016/05/14/msg074674.html
[4] http://mail-index.netbsd.org/source-changes/2016/05/14/msg074675.html
[5] http://mail-index.netbsd.org/source-changes/2016/05/14/msg074677.html
[6] http://mail-index.netbsd.org/source-changes/2016/05/14/msg074678.html