Add a [rtc_]build_with_neon variable to unify conditions.

webrtc/common_audio/BUILD.gn

Index: webrtc/common_audio/BUILD.gn
diff --git a/webrtc/common_audio/BUILD.gn b/webrtc/common_audio/BUILD.gn
index 3d4a3321a0ed68cbe45c9ad9507ba6bddc27a7e0..2f529c5fd35aac3de4e759a0541232fc3bc5b686 100644
--- a/webrtc/common_audio/BUILD.gn
+++ b/webrtc/common_audio/BUILD.gn
@@ -130,14 +130,13 @@ source_set("common_audio") {
     ]
 
     if (arm_version >= 7) {
-      deps += [ ":common_audio_neon" ]
       sources += [ "signal_processing/filter_ar_fast_q12_armv7.S" ]
     } else {
       sources += [ "signal_processing/filter_ar_fast_q12.c" ]
     }
   }
 
-  if (current_cpu == "arm64") {
+  if (rtc_build_with_neon) {
     deps += [ ":common_audio_neon" ]
   }
 
@@ -211,7 +210,7 @@ if (current_cpu == "x86" || current_cpu == "x64") {
   }
 }
 
-if (rtc_build_armv7_neon || current_cpu == "arm64") {
+if (rtc_build_with_neon) {
   source_set("common_audio_neon") {
     sources = [
       "fir_filter_neon.cc",
@@ -221,20 +220,10 @@ if (rtc_build_armv7_neon || current_cpu == "arm64") {
       "signal_processing/min_max_operations_neon.c",
     ]
 
-    configs += [ "..:common_config" ]
+    configs += [
+      "..:arm_neon_config",
+      "..:common_config",
+    ]
     public_configs = [ "..:common_inherited_config" ]
-
-    if (!arm_use_neon) {
-      configs -= [ "//build/config/compiler:compiler_arm_fpu" ]
-      cflags = [ "-mfpu=neon" ]
-    }
-
-    # Disable LTO in audio_processing_neon target due to compiler bug.
-    if (rtc_use_lto) {
-      cflags -= [
-        "-flto",
-        "-ffat-lto-objects",
-      ]
-    }
   }
 }