selftests/nolibc: add test support for ppc

Kernel uses ARCH=powerpc for both 32-bit and 64-bit PowerPC, here adds a
ppc variant for 32-bit PowerPC and uses it as the default variant of
powerpc architecture.

Users can pass XARCH=ppc (or ARCH=powerpc) to test 32-bit PowerPC.

The default qemu-system-ppc g3beige machine [1] is used to run 32-bit
powerpc kernel with pmac32_defconfig. The missing PMACZILOG serial tty
and console are enabled in another patch [2].

Note,

- zImage doesn't boot due to "qemu-system-ppc: Some ROM regions are
  overlapping" error, so, vmlinux is used instead.

- 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.

[1]: https://qemu.readthedocs.io/en/latest/system/ppc/powermac.html
[2]: https://lore.kernel.org/lkml/bb7b5f9958b3e3a20f6573ff7ce7c5dc566e7e32.1690982937.git.tanyuan@tinylab.org/

Suggested-by: Willy Tarreau <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]/
Reviewed-by: Thomas Weißschuh <[email protected]>
Signed-off-by: Zhangjin Wu <[email protected]>
Signed-off-by: Willy Tarreau <[email protected]>

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