Correctly align mSendBuffer to contain doubles.

When mSendBuffer contains struct RS_CMD_ScriptSetVarD_rec, its third
parameter is double, and thus mSendBuffer has to be aligned to the
size of double. Otherwise unaligned read exception occurs on MIPS.

Change-Id: I938f393f7c9cf051842e46d68a05cc99f4062203
diff --git a/rsThreadIO.h b/rsThreadIO.h
index cb7d4ab..e0a108f 100644
--- a/rsThreadIO.h
+++ b/rsThreadIO.h
@@ -89,7 +89,7 @@
     intptr_t mToCoreRet;
 
     size_t mSendLen;
-    uint8_t mSendBuffer[2 * 1024];
+    uint8_t mSendBuffer[2 * 1024] __attribute__((aligned(sizeof(double))));
 
 };