Revert "Fix derp"

This reverts commit 483451340bd4f37d8ca25c455f7f4400a4e75400.
Revert "ARM: 7467/1: mutex: use generic xchg-based implementation for ARMv6+"

This reverts commit cac40f2757168837ef95dd4e8b6f2dcf7d9133a5.