configure: Prevent the NEON check in configure from passing under aarch64.

The test verifies that the NEON C intrinsics work, but the rest of the
codebase uses lots of direct ARMv7 NEON assembly. The same intrinsics
work in A64, but the assembly is slightly different.

Prevent the check from passing so that we don't use this where it won't
work.

https://bugzilla.gnome.org/show_bug.cgi?id=712367

configure.ac

@@ -190,6 +190,7 @@ AC_LINK_IFELSE([
 AC_LANG_PROGRAM([[
   #include <arm_neon.h>
   int32x4_t testfunc(int16_t *a, int16_t *b) {
+      asm volatile ("vmull.s16 q0, d0, d0" : : : "q0");
       return vmull_s16(vld1_s16(a), vld1_s16(b));
   }
 ]])],