Draft PR for in-progress work to accelerateSystem.Half with FP16 ISA.

Current work done:

1. Add aTYP_HALF to the .NET runtime, which is treated like aTYP_SIMDXX, but with some notable differences. Namely, aTYP_HALF is passed around via the xmm registers, and while it will pass avarTypeIsStruct test, it must be treated as a primitive in other places.

2. AccelerateSystem.Half operations with theTYP_HALF and some FP16 intrinsics. Not every possible function has been accelerated yet.

For discussion:

1. I have currently worked around some checks to makeTYP_HALF behave like a struct and a primitive. It's very ad-hoc at the moment.

2. Much of the work to transform the namedSystem.Half intrinsics into a sequence of intrinsic nodes is done inimportcall.cpp and might want to be moved up into some of thegtNewSimdXX nodes.