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}
1 file changed