Remove the requirement that Subzero must not diverge from the upstream
repo.

Recommend using the MAKE_BUILD_PARALLEL_LEVEL environment variable to
change the default parallelism.

Fixes: b/166244497
Fixes: b/170565506
Change-Id: I00029e392282edd650712f031079073482365af3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51188
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>