[PowerPC] Extend and update the test for half support (NFC)

`f16` is more functional than just a storage type on the platform,
though it does have some codegen issues [1]. To prepare for future
changes, do the following nonfunctional updates to the existing `half`
test:

* Add tests for passing and returning the type directly.
* Add tests showing bitcast behavior, which is currently incorrect but
  serves as a baseline.
* Add tests for `fabs` and `copysign` (trivial operations that shouldn't
  require libcalls).
* Add invocations for big-endian and for PPC32.
* Rename the test to `half.ll` to reflect its status, which also matches
  other backends.

[1]: #97975

Author: tgross35