The typedefs for Atomic32 and Atomic64 were sometimes causing
Atomic32 to be an int32 rather than an intptr_t on 32-bit platforms.
On some of these platforms (ARM/CortexM building with GCC 6 in one
case) int32 is a long int, while intptr_t is an int, which causes a
compiler error even though long int and int are both 4 bytes.

Having Atomic32 always be intptr_t on 32-bit platforms and Atomic64
always be intptr_t on 64-bit platforms should resolve any of these
types of errors.