selftests/vm: skip 128TBswitch on unsupported arch

The test va_128TBswitch.c exercises a feature only supported on PPC and
x86_64, but it's run on other 64-bit archs as well.  Before this patch,
the test did nothing and returned 0 for KSFT_PASS.  This patch makes it
return the KSFT codes from kselftest.h, including KSFT_SKIP when
appropriate.

Verified on arm64 and x86_64.

Link: https://lkml.kernel.org/r/20220704123813.427625-1-adam@wowsignal.io


Signed-off-by: Adam Sindelar <adam@wowsignal.io>
Cc: David Vernet <void@manifault.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>