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.
(cherry picked from commit bad7a2dd0b6758da05572dba4652b599ff621571)

configure.ac

@@ -1381,7 +1381,10 @@ 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"
     ], [