selftests/mm: fix powerpc ARCH check

In commit 0518dbe9 ("selftests/mm: fix cross compilation with LLVM")
the logic to detect the machine architecture in the Makefile was changed
to use ARCH, and only fallback to uname -m if ARCH is unset.  However the
tests of ARCH were not updated to account for the fact that ARCH is
"powerpc" for powerpc builds, not "ppc64".

Fix it by changing the checks to look for "powerpc", and change the
uname -m logic to convert "ppc64.*" into "powerpc".

With that fixed the following tests now build for powerpc again:
 * protection_keys
 * va_high_addr_switch
 * virtual_address_range
 * write_to_hugetlbfs

Link: https://lkml.kernel.org/r/20240506115825.66415-1-mpe@ellerman.id.au
Fixes: 0518dbe9

 ("selftests/mm: fix cross compilation with LLVM")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mark Brown <broonie@kernel.org>
Cc: <stable@vger.kernel.org>	[6.4+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>