LLVMJIT: Remove the TargetMachine cache.
LLVMJIT: Remove the TargetMachine cache.

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/+/44013
Reviewed-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader