Do without union of variable length arrays.

Do without union of variable length arrays.

Prepare to compile with clang.

A union like
  { T32 a32[n]; T64 a64[n]; } u;
is expanded to
  size_t nbytes = n * MAX(sizeof(T32), sizeof(T64));
  void *data = malloc(nbytes);
  T32 (*a32)[n] = data;
  T64 (*a64)[n] = data;

Signed-off-by: Chih-Hung Hsieh <chh@google.com>