[SystemZ] ABI support for single-element vector types

Support passing and returning values of single-element vector
types (i.e. <1 x i128> and <1 x fp128>).

Now that i128 is a legal type, supporting these types can be
done simply by providing a getRegisterTypeForCallingConv
implementation that handles them.

Fixes https://github.com/llvm/llvm-project/issues/61291