Stop SwiftShader building if enable_swiftshader=false

Swiftshader doesn't build at this time for Windows on Arm,
and all WoA devices are new enough not to need it at the
moment.

Change-Id: I5587ab1a324930aaac1b31301f39e3da8b7fabe6
Reviewed-on: https://chromium-review.googlesource.com/c/1409453
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622848}
1 file changed