Remove OSReadLittle* due to alignment requirements (#6678)

The OSReadLittleInt64 function as defined by Apple reduces down to:

`return *(volatile uint64_t *)((uintptr_t)base + byteOffset);`

which means we are type-punning using a cast. On ARMv7 and other aligned architectures this can cause crashes. 
Minimal example: https://gist.github.com/dmaclach/b10b0a71ae614d304c067cb9bd264336

Fixes #6679

objectivec/GPBCodedInputStream.m

@@ -93,14 +93,22 @@ static int8_t ReadRawByte(GPBCodedInputStreamState *state) {
 
 static int32_t ReadRawLittleEndian32(GPBCodedInputStreamState *state) {
   CheckSize(state, sizeof(int32_t));
-  int32_t value = OSReadLittleInt32(state->bytes, state->bufferPos);
+  // Not using OSReadLittleInt32 because it has undocumented dependency
+  // on reads being aligned.
+  int32_t value;
+  memcpy(&value, state->bytes + state->bufferPos, sizeof(int32_t));
+  value = OSSwapLittleToHostInt32(value);
   state->bufferPos += sizeof(int32_t);
   return value;
 }
 
 static int64_t ReadRawLittleEndian64(GPBCodedInputStreamState *state) {
   CheckSize(state, sizeof(int64_t));
-  int64_t value = OSReadLittleInt64(state->bytes, state->bufferPos);
+  // Not using OSReadLittleInt64 because it has undocumented dependency
+  // on reads being aligned.  
+  int64_t value;
+  memcpy(&value, state->bytes + state->bufferPos, sizeof(int64_t));
+  value = OSSwapLittleToHostInt64(value);
   state->bufferPos += sizeof(int64_t);
   return value;
 }