Explicitly set target_subarch for iOS.
Explicitly set target_subarch for iOS.

WebRTC doesn't use or support the target_subarch
GYP variable in the same way as Chromium does, and we
currently don't build fat binaries at our buildbots.

This is needed to unblock the chromium_revision roll in
https://webrtc-codereview.appspot.com/50569004/ and it
also makes it a bit easier to build for iOS since it
is no longer needed to set target_subarch=arm64 when
target_arch=arm64 (i.e. when you build for ARM64).

TBR=tkchin@webrtc.org
BUG=4503

Review URL: https://webrtc-codereview.appspot.com/48869004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8937}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f6b7265c6bd254b1558b8e04a2c6d2c1769ef4a6

Alkalyne/webrtctrunk,krieger-od/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,jchavanton/webrtc,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,sippet/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,sippet/webrtc,krieger-od/webrtc,aleonliao/webrtc-trunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,krieger-od/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,krieger-od/webrtc,sippet/webrtc,Alkalyne/webrtctrunk,sippet/webrtc,sippet/webrtc,krieger-od/webrtc,sippet/webrtc,krieger-od/webrtc,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,jchavanton/webrtc