If the Reactor backend is Subzero, there's no need to build LLVM. This
is accomplished by excluding the LLVM target from the "all" target.

Bug: b/151250656
Change-Id: I26dc44921cc44d379c619700c6dd03f6fe94ee27
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42229
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>