arm64: Create an L3 table to limit permissions

When building a 4k page kernel we use 2M blocks to map the kernel
contents. As the .text section may not end on a 2M aligned address
we need to split one block into level 3 pages and pad the end of the
section to an appropriate boundary.

With both these changes we can then mapjust the code as executable.
While here also map it as read-only as none ofthis shouldbe written
to directly.

Reviewed by: alc
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45064