Switch to Subzero instead of LLVM on Linux.

Switch to Subzero instead of LLVM on Linux.

Change-Id: Ib13af2fb929df98e0bb0e5af33a935942d854706
Reviewed-on: https://swiftshader-review.googlesource.com/9008
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>
(cherry picked from commit 2aeaa6f445b1878a1ee3986c3b08ffa50323b366)
Reviewed-on: https://swiftshader-review.googlesource.com/9108
Reviewed-by: Alexis Hétu <sugoi@google.com>