ARM: check ARMv6 support at the same time as NEON

Our NEON optimizations don't compile on ARMv5 and lower. It is
dubious that there would be any use for NEON on such processor anyway.

configure.ac

@@ -1351,7 +1351,10 @@ AS_IF([test "${enable_neon}" != "no"], [
   AC_CACHE_CHECK([if $CC groks NEON inline assembly], [ac_cv_neon_inline], [
     CFLAGS="${CFLAGS_save} -mfpu=neon"
     AC_COMPILE_IFELSE([
-      AC_LANG_PROGRAM(,[[asm volatile("vqmovun.s64 d0, q1":::"d0");]])
+      AC_LANG_PROGRAM(,[[
+asm volatile("vqmovun.s64 d0, q1":::"d0");
+asm volatile("qadd r0, r0, r0":::"r0"); /* assume ARMv6 */
+]])
     ], [
       ac_cv_neon_inline="-mfpu=neon"
     ], [