Set asmflags when building for iOS.

It's probably best we build iOS assembly with the iOS SDK and the right
architecture settings in Clang...

Change-Id: I68119641b1f86267ca85434b5a7f6ed37838ce23
Reviewed-on: https://skia-review.googlesource.com/8680
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gn/BUILD.gn

@@ -190,6 +190,12 @@ config("default") {
     if (target_cpu == "arm") {
       _target = "armv7"
     }
+    asmflags += [
+      "-isysroot",
+      ios_sysroot,
+      "-arch",
+      _target,
+    ]
     cflags += [
       "-isysroot",
       ios_sysroot,