vm: Remove kernel stack swapping support, part 8

• The kernel stack objects do not need to be pageable, so use OBJT_PHYS objects instead. The main difference is that mappings do not require PV entries.
• Make some externally visible functions, relating to kernel thread stack internals, private to vm_glue.c, as their external consumers are now gone.

Tested by: pho
Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D46119