Revert "ARM: mutex: use generic atomic_dec-based implementation for ARMv6+"

This reverts commit 21fe6e2f261763aab49750dcaca6bbe405c4bbae.
Revert "ARM: 7467/1: mutex: use generic xchg-based implementation for ARMv6+"

This reverts commit b49a23fc4bab3f2c2fd7dfc05097ffe8c9e9c512.