This is done so that we can build for both the subzero and llvm backends.

Old configs remain where they are during a transition period.

Bug: b/130343040
Change-Id: Icc9e1b92074ef90aacfd9e6d40b021d35f4d9ef3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28908Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>