MFC r281495:

MFC r281495:
Add config option PAE_TABLES for the i386 kernel.  It switches pmap to
use PAE format for the page tables, but does not incur other
consequences of the full PAE config.  In particular, vm_paddr_t and
bus_addr_t are left 32bit, and max supported memory is still limited
by 4GB.

The option allows to have nx permissions for memory mappings on i386
kernel, while keeping the usual i386 KBI and avoiding the kernel data
sizing problems typical for the PAE config.

git-svn-id: svn://svn.freebsd.org/base/stable/10@282065 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f