arm: Fix building neon txfm with clang
arm: Fix building neon txfm with clang

This fixes compiler errors like these with clang (any version):

error: argument to '__builtin_neon_vqrshrn_n_v' must be a constant integer
    low = vqrshrn_n_s32(out_low, (int32_t)NewSqrt2Bits);
          ^                      ~~~~~~~~~~~~~~~~~~~~~

Even though NewSqrt2Bits was a static const int, the clang semantic
analyze pass doesn't deem it constant enough for this context. (For
reference, in C++ mode, it would accept this as a suitable constant
though.)

Change-Id: I9ae2281ad425c80376ad0433143c26fcd3006874

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom