Moving asm code out of isac_fix_c sources list

BUG=webrtc:7743

Review-Url: https://codereview.webrtc.org/2973613002
Cr-Commit-Position: refs/heads/master@{#18911}
diff --git a/webrtc/modules/audio_coding/BUILD.gn b/webrtc/modules/audio_coding/BUILD.gn
index ce775dc..d5b669c 100644
--- a/webrtc/modules/audio_coding/BUILD.gn
+++ b/webrtc/modules/audio_coding/BUILD.gn
@@ -634,6 +634,16 @@
   public_configs = [ ":isac_fix_config" ]
 }
 
+rtc_source_set("isac_fix_c_arm_asm") {
+  sources = []
+  if (current_cpu == "arm" && arm_version >= 7) {
+    sources += [
+      "codecs/isac/fix/source/lattice_armv7.S",
+      "codecs/isac/fix/source/pitch_filter_armv6.S",
+    ]
+  }
+}
+
 rtc_source_set("isac_fix_c") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
   sources = [
@@ -702,14 +712,11 @@
   }
 
   if (current_cpu == "arm" && arm_version >= 7) {
-    sources += [
-      "codecs/isac/fix/source/lattice_armv7.S",
-      "codecs/isac/fix/source/pitch_filter_armv6.S",
-    ]
     sources -= [
       "codecs/isac/fix/source/lattice_c.c",
       "codecs/isac/fix/source/pitch_filter_c.c",
     ]
+    deps += [ ":isac_fix_c_arm_asm" ]
   }
 
   if (current_cpu == "mipsel") {