Move target_subarch from gyp_webrtc to supplement.gypi

Since the webrtc/build/supplement.gypi file is loaded before
Chromium's build/common.gypi it is the right place to override
GYP variables. This makes it possible to use the approach in
https://webrtc-codereview.appspot.com/48869004, which I applied
to supplement.gypi in this CL, instead of the hacky approach in
https://webrtc-codereview.appspot.com/48909004 (removed in this CL).

R=tkchin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/50729004

Cr-Commit-Position: refs/heads/master@{#9017}
2 files changed