kernel: Break thread-related AutoLockers into a separate header.

Including thread.h brings a massive array of things with it from
the kernel thread arch headers, team and thread definitions,
hash tables, linked lists, Referenceable, etc. that the vast majority
of AutoLock.h consumers neither want nor need.

So, put these in a separate header, and adjust all consumers of these
lockers to include the new file.

This change exposes the fact that a lot of files were inadvertently
making use of headers included indirectly through thread.h. Those
will be fixed in the next commit.

Fix ppc build

Remaining uses of PAGE_SIZE that I didn't notice.

PPC: Restructure paging stuff to match other platforms

First attempt.

Totally untested.