Only use the ARM NEON 32-way unrolled rANS on AArch64.

NEON alone isn't a sufficient guard as AArch32 also has some limited
Neon capabilities.  While we could no doubt have a 32-bit alternative,
for now this is the simple fix and let aarch32 use the scalar
implementation.

Doing a 32-bit neon is a complex task and without having access to the
hardware it's pretty much impossible.  I also wouldn't have high hopes
for any significant speed gains over scalar with only half the lanes
available.

Fixes samtools#81

htscodecs/rANS_static32x16pr_neon.c

@@ -32,7 +32,7 @@
  */
 
 #include "config.h"
-#ifdef __ARM_NEON
+#if defined(__ARM_NEON) && defined(__aarch64__)
 #include <arm_neon.h>
 
 #include <limits.h>