Clean up typedefs for Atomic32/Atomic64

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.

src/google/protobuf/stubs/atomicops.h

@@ -63,17 +63,11 @@ namespace google {
 namespace protobuf {
 namespace internal {
 
-#if defined(GOOGLE_PROTOBUF_ARCH_POWER)
+#ifdef GOOGLE_PROTOBUF_ARCH_32_BIT
-#if defined(_LP64) || defined(__LP64__)
-typedef int32 Atomic32;
-typedef intptr_t Atomic64;
-#else
   typedef intptr_t Atomic32;
   typedef int64 Atomic64;
-#endif
 #else
   typedef int32 Atomic32;
-#ifdef GOOGLE_PROTOBUF_ARCH_64_BIT
   // We need to be able to go between Atomic64 and AtomicWord implicitly.  This
   // means Atomic64 and AtomicWord should be the same type on 64-bit.
   #if defined(__ILP32__) || defined(GOOGLE_PROTOBUF_OS_NACL)
@@ -85,7 +79,6 @@ typedef int64 Atomic64;
   typedef intptr_t Atomic64;
   #endif
 #endif
-#endif
 
 // Use AtomicWord for a machine-sized pointer.  It will use the Atomic32 or
 // Atomic64 routines below, depending on your architecture.