And add the LLVM_VERSION env var and plumb this into the shell scripts.
Stub building the 10.0 builds for now. We'll enable them one at a time.

Bug: b/152339534
Change-Id: I645a6d748dec956505ba550ce96ff85a21f9f73e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43012Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>