LLVMReactor used to have a Big Fat Global Mutex over the entire
lifetime of the Nucleus object. This was required as LLVMReactor
used global variables for storing builder state.

Over the past year, there has been significant code cleanup and
global state has been reduced to a couple of globals that can now
be marked thread_local. With all state now being immutable global
or thread local, we are now able to remove the mutex.

ASAN and TSAN checks for our unittests are clean.

Bug: b/153803432
Change-Id: Ibe4019fb783f86e734387db431539e915369b488
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33484Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>