AGC2: Fix GetAvailableCpuFeatures()

Handle the case in which neither WEBRTC_ARCH_X86_FAMILY nor
WEBRTC_HAS_NEON are defined.

Bug: webrtc:10480
Change-Id: I241583911d8e5645dfbd39b60337dd20b2d9f046
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/196525
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32786}

modules/audio_processing/agc2/cpu_features.cc

@@ -48,6 +48,10 @@ AvailableCpuFeatures GetAvailableCpuFeatures() {
   return {/*sse2=*/false,
           /*avx2=*/false,
           /*neon=*/true};
+#else
+  return {/*sse2=*/false,
+          /*avx2=*/false,
+          /*neon=*/false};
 #endif
 }