Also set the Subzero ones to use 10.0. This is only necessary so that
CMake is able to add the llvm-10.0 subdirectory, despite the target not
being built.

Bug: b/152339534
Change-Id: Ieacd6cff8559c07370a311a766e2d9a28f8dc73a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46749
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>