arch: don't call *Timing functions from *Atomic versions

arch: don't call *Timing functions from *Atomic versions

The readMemAtomic/writeMemAtomic helper functions were calling
readMemTiming/writeMemTiming respectively.  This is functionally
correct, since the *Timing functions are doing the same access
initiation operation as the *Atomic functions (just that the
*Atomic versions also complete the access in line).  It also
provides for some (very minimal) code reuse.  Unfortunately,
it's potentially pretty confusing, since it makes it look like
the atomic accesses are somehow being converted to timing
accesses.  It also gets in the way of specializing the timing
interface (as will be done in a future patch).