mbed_atomic_impl.h: Add pre-op (xxx_fetch) forms

Even though C/C++11 don't offer pre-op forms (that do the operation then
return the new value) of their freestanding functions, we will be making
them visible via pre-op operators on `Atomic<T>` (++, --, +=, -=, &=,
|=, ^=).

It is easier to do a pre-op than a post-op, as we can use one
fewer register in the assembler, so it's worth optimising for what will
be quite common cases. Make these forms accessible for `Atomic<T>`, but
don't document them for standalone use at this stage.

Author: kjbracey