Squash revert of 3 commits that were breaking build.

Revert "ARM: Change dmb domain for bionic_atomic_barrier()"

This reverts commit a5e538d12a40ef65eabf230d46f4602df305e79d.

Revert "Clean up the pthread-only atomic stuff a little."

This reverts commit a74a1ac0dc7fb8375e53c7e4d5f6cd468daeeb42.

Revert "Clean up some ARMv4/ARMv5 cruft."

This reverts commit e0a742ad46e73590a2115cfdb47bcd087ace5fd5.

Change-Id: I5bed5e6441562d565b5829582a806836d6ec5c7a