PPC NCG: Test `AtomicFetch` panics on PowerPC

Summary

ghc-9.7.20230407: panic! (the 'impossible' happened)
  GHC version 9.7.20230407:
	PPC.Ppr.Instr LDR: no match
  CallStack (from HasCallStack):
    panic, called at compiler/GHC/CmmToAsm/PPC/Ppr.hs:411:22 in ghc:GHC.CmmToAsm.PPC.Ppr
Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
*** unexpected failure for AtomicFetch(normal)

Steps to reproduce

hadrian/build test --only=AtomicFetch

Expected behavior

Test passes.

Environment

• GHC version used: 9.7.20230407

Optional:

• Operating System: Linux
• System Architecture: powerpc64le