TargetMachine have internal fields mutated during compilation.
In order to support concurrent implementation, each compile needs a new TargetMachine.

Bug: b/153803432
Change-Id: I1a46d8e0193e6176c777d82ce10008afca91ec94
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44013Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>