Replace stray reference to cpu_arch in BUILD.gn. cpu_arch has been replaced by target_cpu and current_cpu in GN.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/260971 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>