On platforms where the calling thread has a very small stack, LLVM can
overflow the stack. Work around it by creating a temporary thread for
performing LLVM optimizations and codegen.

Bug: b/149829034
Change-Id: I59edd0f7d02b4724ea30f9413ea3efbb4d200290
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41389Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>