powerpc/xmon: Reapply "Relax frame size for clang"

powerpc/xmon: Reapply "Relax frame size for clang"

This is a manual revert of commit 7f3c5d099b6f ("Revert "powerpc/xmon:
Relax frame size for clang"") but using ccflags-$(CONFIG_CC_IS_CLANG)
which is shorter.

Turns out that this is reproducible still under specific compiler
versions (mea culpa: I did not test every supported version of clang),
and even a few randconfigs bots found.

We'll have to revisit this again in the future, for now back this out.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://github.com/ClangBuiltLinux/linux/issues/252#issuecomment-1690371256
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/llvm/202308260344.Vc4Giuk7-lkp@intel.com/
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230828-ppc_rerevert-v2-1-46b71a3656c6@google.com