Apple's Clang can't assemble jsimd_arm_...

This should fix Build-Mac-Clang-arm64-Debug-GN_iOS_NoBuildbot.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4755

Change-Id: Ie49ce5642fb2d373102c9309074e13ee3035a569
Reviewed-on: https://skia-review.googlesource.com/4755
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/third_party/libjpeg-turbo/BUILD.gn b/third_party/libjpeg-turbo/BUILD.gn
index 927df49..6825b6b 100644
--- a/third_party/libjpeg-turbo/BUILD.gn
+++ b/third_party/libjpeg-turbo/BUILD.gn
@@ -64,12 +64,12 @@
       "../externals/libjpeg-turbo/jutils.c",
     ]
 
-    if (current_cpu == "arm") {
+    if (current_cpu == "arm" && current_os != "ios") {
       sources += [
         "../externals/libjpeg-turbo/simd/jsimd_arm.c",
         "../externals/libjpeg-turbo/simd/jsimd_arm_neon.S",
       ]
-    } else if (current_cpu == "arm64") {
+    } else if (current_cpu == "arm64" && current_os != "ios") {
       sources += [
         "../externals/libjpeg-turbo/simd/jsimd_arm64.c",
         "../externals/libjpeg-turbo/simd/jsimd_arm64_neon.S",