[x86] Synchronize fp128 and i128 ABI tests (nfc) (#147923)
The ABI tests for fp128 covers a handful of things that the i128 test
currently does not, including targets other than x86-64 Linux and some
simpler argument cases. Update the i128 test to be consistent with
fp128. All existing test function patterns still exist, though they all
wind up renamed.
Additionally correct some `call` signatures in the fp128 test (not
relevant for output).
