selftests/nolibc: add test support for ppc64

Kernel uses ARCH=powerpc for both 32-bit and 64-bit PowerPC, here adds a
ppc64 variant for big endian 64-bit PowerPC, users can pass XARCH=ppc64
to test it.

The powernv machine of qemu-system-ppc64 is used with
powernv_be_defconfig.

As the document [1] shows:

  PowerNV (as Non-Virtualized) is the “bare metal” platform using the
  OPAL firmware. It runs Linux on IBM and OpenPOWER systems and it can be
  used as an hypervisor OS, running KVM guests, or simply as a host OS.

Notes,

- differs from little endian 64-bit PowerPC, vmlinux is used instead of
  zImage, because big endian zImage [2] only boot on qemu with x-vof=on
  (added from qemu v7.0) and a fixup patch [3] for qemu v7.0.51:

- since the VSX support may be disabled in kernel side, to avoid
  "illegal instruction" errors due to missing VSX kernel support, let's
  simply let compiler not generate vector/scalar (VSX) instructions via
  the '-mno-vsx' option.

- as 'man gcc' shows, '-mmultiple' is used to generate code that uses
  the load multiple word instructions and the store multiple word
  instructions. Those instructions do not work when the processor is in
  little-endian mode (except PPC740/PPC750), so, we only enable it
  for big endian powerpc.

- for big endian ppc64, as the help message from arch/powerpc/Kconfig
  shows, the V2 ABI is standard for 64-bit little-endian, but for
  big-endian it is less well tested by kernel and toolchain, so, use
  elfv1 as-is, no need to explicitly ask toolchain to use elfv2 here.

[1]: https://qemu.readthedocs.io/en/latest/system/ppc/powernv.html
[2]: https://github.com/linuxppc/issues/issues/402
[3]: https://lore.kernel.org/qemu-devel/20220504065536.3534488-1-aik@ozlabs.ru/

Suggested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/lkml/20230722121019.GD17311@1wt.eu/
Link: https://lore.kernel.org/lkml/20230719043353.GC5331@1wt.eu/
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>

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