[Arm64EC] Add support for `fp128` (#206980)
fixes https://github.com/llvm/llvm-project/issues/94434
Analogue to https://github.com/llvm/llvm-project/pull/152843 but for
`fp128`.
Testing is a bit thin still, we need to fix some other problems like
https://github.com/llvm/llvm-project/issues/144006 so that the arm64ec
target can be added to more tests.
