squashed: revert recent armv6 additions

Revert "Implement armv6 optimized strlen from glibc 2.18"

This reverts commit eeaf61d9771b6268353f04690d3378b71e723fa5.

Revert "armv6: REX routines are currently broken, use alternatives"

This reverts commit 1e7aa69987cb322a9000ca400d05e8c1b00f7580.

Revert "Revert "bionic_atomic_arm.h: Remove < ARMv6 support""

This reverts commit fa51494b5d5d1518b352608abdfaedf8b579865a.

Change-Id: Ib7e489eae623f061d1b50af672c101929846ed8f