Explicitly set target_subarch for iOS (re-land)
Explicitly set target_subarch for iOS (re-land)

Since the approach in https://webrtc-codereview.appspot.com/48869004/
was not possible, this is the only way to set
the target_subarch GYP_DEFINES 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).

BUG=4503
TESTED=Ran successful project generations using:
GYP_DEFINES="OS=ios target_arch=arm" webrtc/build/gyp_webrtc
and verified the -arch compiler flag was set to 'armv7'

GYP_DEFINES="OS=ios target_arch=arm64" webrtc/build/gyp_webrtc
and verified the -arch compiler flag was set to 'arm64'

R=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48909004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8949}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc