Update the `EmitterUnitTests` to support an alternative disassembler

Currently the emitter has a set of unit tests (genAmd64EmitterUnitTests and genArm64EmitterUnitTests) which can be used for validating that the emitted byte code is correct for various scenarios.

However, this does not currently work in CoreCLR itself due to a dependency on the closed source msvcdis disassembler.

Given the number of new instructions being supported and the number of general improvements being made to the emitter for the Hardware Intrinsics feature, it would be beneficial to update these tests such that they can be run as part of CoreCLR.

category:correctness theme:testing skill-level:intermediate cost:medium impact:small