[llvm] [SystemZ] Test improvements for atomic load/store instructions (NFC). (PR #75630)

[Jonas Paulsson via llvm-commits]

JonPsson1 wrote:

Per our discussion, I reverted the previous test changes for <=64 bits, but updated the 128-bit tests as they actually should have been changed with c568927, since the default alignment for _Atomic long double is now 16 bytes, right?


https://github.com/llvm/llvm-project/pull/75630