cpu_arch has been replaced by target_cpu and current_cpu in GN.

R=geofflang@chromium.org
BUG=344767

Change-Id: Id1c0e37238db5fac41572244aeac0908ac5a412a
Reviewed-on: https://chromium-review.googlesource.com/260971Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>