Add iOS simulator support in GN.

This builds fine on my laptop, but fails to link on the bots,
so I'm leaving out a new Build bot for now.

BUG=skia:6329

Change-Id: I4b33770f13ab9dec914d090b45d9921b19ee2c9d
Reviewed-on: https://skia-review.googlesource.com/9519
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 5ccc729..7741354 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -50,7 +50,11 @@
 }
 
 if (is_ios) {
-  ios_sysroot = exec_script("find_ios_sysroot.py", [], "trim string")
+  sdk = "iphoneos"
+  if (target_cpu == "x86" || target_cpu == "x64") {
+    sdk = "iphonesimulator"
+  }
+  ios_sysroot = exec_script("find_ios_sysroot.py", [ sdk ], "trim string")
 }
 
 config("default") {
@@ -190,6 +194,10 @@
     _target = target_cpu
     if (target_cpu == "arm") {
       _target = "armv7"
+    } else if (target_cpu == "x86") {
+      _target = "i386"
+    } else if (target_cpu == "x64") {
+      _target = "x86_64"
     }
     asmflags += [
       "-isysroot",