Set the MALLOC_ALIGNMENT to 16.

This isn't strictly required, as wasm SIMD loads and stores work on
unaligned memory. However, it may provide better performance. That said,
this isn't currently studied by any benchmarking.

Author: sunfishcode

dlmalloc/src/dlmalloc.c

@@ -29,6 +29,9 @@
 #define NO_MALLINFO 1
 #define NO_MALLOC_STATS 1
 
+/* Align malloc regions to 16, to avoid unaligned SIMD accesses. */
+#define MALLOC_ALIGNMENT 16
+
 /*
  * Declare errno values used by dlmalloc. We define them like this to avoid
  * putting specific errno values in the ABI.