arm: assume NEON if built with it

Downstream may build with -march=armv7-a and/or -mfpu=neon or have
those enabled by default e.g., on FreeBSD armv7. There's no point
detecting NEON then only to fail if OS doesn't support getauxval().

src/arm/cpu.c

@@ -72,6 +72,8 @@ unsigned dav1d_get_cpu_flags_arm(void) {
     unsigned flags = 0;
 #if ARCH_AARCH64
     flags |= DAV1D_ARM_CPU_FLAG_NEON;
+#elif defined(__ARM_NEON)
+    flags |= DAV1D_ARM_CPU_FLAG_NEON;
 #elif defined(HAVE_GETAUXVAL) && ARCH_ARM
     unsigned long hw_cap = getauxval(AT_HWCAP);
     flags |= (hw_cap & NEON_HWCAP) ? DAV1D_ARM_CPU_FLAG_NEON : 0;