Modifications to update.py:
* The script now clones and builds llvm in a temporary location based on
  the LLVM_BRANCH and LLVM_COMMIT variables in the script.
* Added logging, including for execution of commands
* Now setting LLVM_ENABLE_THREADS=ON as we recently manually enabled
  multithreading for the LLVM backend.

Change-Id: I53bed46d890c0da234c4245d456c83280bf7b370
Bug: b/152339534
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46188Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>