Implement CPU feature detection for ARM Linux.

webrtc/system_wrappers/BUILD.gn

Index: webrtc/system_wrappers/BUILD.gn
diff --git a/webrtc/system_wrappers/BUILD.gn b/webrtc/system_wrappers/BUILD.gn
index e7892605317a8fab23b995c4b41ddbbdae4e55bb..c2b370c4cc2faadfa3627087a4dcefd07669d67f 100644
--- a/webrtc/system_wrappers/BUILD.gn
+++ b/webrtc/system_wrappers/BUILD.gn
@@ -108,6 +108,10 @@ static_library("system_wrappers") {
   if (is_linux) {
     defines += [ "WEBRTC_THREAD_RR" ]
 
+    if (!build_with_chromium) {
+      deps += [ ":cpu_features_linux" ]
+    }
+
     libs += [ "rt" ]
   }
 
@@ -178,3 +182,14 @@ if (is_android) {
     ]
   }
 }
+
+if (is_linux) {
+  source_set("cpu_features_linux") {
+    sources = [
+      "source/cpu_features_linux.c",
+    ]
+
+    configs += [ "..:common_config" ]
+    public_configs = [ "..:common_inherited_config" ]
+  }
+}