Support the arm64 pmap_remove_write for stage 2

[andrew]

Summary

The fields we need to adjust are different in stage 1 and stage 2
tables. Handle this by adding variables to hold the bits to check,
set, and clear.

[kib]

You might take a look at PG_RW/PG_V, pmap_valid_bit()/pmap_rw_bit() in amd64 pmap.

[alc]

sys/arm64/arm64/pmap.c

5876

This is incorrect. ATTR_S1_AP(ATTR_S1_AP_RW) is equal to 0. So, the original code calls vm_page_dirty() if (oldpte & ATTR_S1_AP_RW_BIT) == 0.

[andrew]

Fix the check before calling vm_page_dirty