Explicitly set target_subarch for iOS on ia32/x64

https://webrtc-codereview.appspot.com/48909004/ only fixed
the target_subarch problem for ARM. This fixes it for the
x86 targets that are used for the iOS simulator (ia32/x64).

BUG=4532
TESTED=Built locally using:
export GYP_DEFINES="build_with_libjingle=1 build_with_chromium=0 libjingle_objc=1 OS=ios target_arch=ia32"
export GYP_GENERATOR_FLAGS="$GYP_GENERATOR_FLAGS output_dir=out_sim"
export GYP_CROSSCOMPILE=1
gclient runhooks
ninja -C out_sim/Release-iphonesimulator iossim AppRTCDemo

R=tkchin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44159004

Cr-Commit-Position: refs/heads/master@{#8991}
diff --git a/webrtc/build/gyp_webrtc b/webrtc/build/gyp_webrtc
index b96b2d8..5945b95 100755
--- a/webrtc/build/gyp_webrtc
+++ b/webrtc/build/gyp_webrtc
@@ -94,9 +94,9 @@
   if gyp_vars.get('OS') == 'ios' and 'target_subarch' not in gyp_vars:
     os.environ.setdefault('GYP_DEFINES', '')
     target_subarch = None
-    if gyp_vars.get('target_arch') == 'arm' :
+    if gyp_vars.get('target_arch') in ('arm', 'ia32'):
       target_subarch = 'arm32'
-    elif gyp_vars.get('target_arch') == 'arm64':
+    elif gyp_vars.get('target_arch') in ('arm64', 'x64'):
       target_subarch = 'arm64'
     if target_subarch:
       print 'INFO: Appending target_subarch=%s to GYP_DEFINES' % target_subarch