powerpc: replace tailq pointers with iterators

[dougm]

Summary

Change architecture-specific code to use iterators rather than tailq pointers.

[kib]

sys/powerpc/aim/mmu_oea64.c

3802

I am curious why this condition is needed now.

[dougm]

sys/powerpc/aim/mmu_oea64.c

3802

make buildkernel TARGET=powerpc TARGET_ARCH=powerpc will emit these lines:

--- mmu_oea64.o ---
/freebsd/dougm/src/sys/powerpc/aim/mmu_oea64.c:3804:1: warning: unused function 'moea64_sp_promote' [-Wunused-function]
 3804 | moea64_sp_promote(pmap_t pmap, vm_offset_t va, vm_page_t m)
      | ^~~~~~~~~~~~~~~~~

If the condition is removed.

[kib]

sys/powerpc/aim/mmu_oea64.c

3802

Ok, it is fine, but should be a separate commit.

[dougm]

Add braces. Drop a null check. Restore a physical address check. Drop attempts to silence compiler warnings.