selftest/mm: va_high_addr_switch: add ppc64 support check

selftest/mm: va_high_addr_switch: add ppc64 support check

Add PPC64 Radix MMU support to the va_high_addr_switch.sh by introducing
check_supported_ppc64().  The function verifies:

  - 5-level paging (PGTABLE_LEVELS >= 5) enable in kernel config
  - Radix MMU (required for PPC64 5-level translation)
  - HugePages availability (needed for some tests)

If any check fails, the test is skipped (ksft_skip).  This ensures
compatibility with Power9/Power10 systems running in Radix MMU mode.

Avoid failures on 4-level paging system:

  # mmap(NULL, MAP_HUGETLB): 0xffffffffffffffff - FAILED
  # mmap(LOW_ADDR, MAP_HUGETLB): 0xffffffffffffffff - FAILED
  # mmap(HIGH_ADDR, MAP_HUGETLB): 0xffffffffffffffff - FAILED
  # mmap(HIGH_ADDR, MAP_HUGETLB) again: 0xffffffffffffffff - FAILED
  # mmap(HIGH_ADDR, MAP_FIXED | MAP_HUGETLB): 0xffffffffffffffff - FAILED
  # mmap(-1, MAP_HUGETLB): 0xffffffffffffffff - FAILED
  # mmap(-1, MAP_HUGETLB) again: 0xffffffffffffffff - FAILED
  # mmap(ADDR_SWITCH_HINT - PAGE_SIZE, 2*HUGETLB_SIZE, MAP_HUGETLB): 0xffffffffffffffff - FAILED
  # mmap(ADDR_SWITCH_HINT , 2*HUGETLB_SIZE, MAP_FIXED | MAP_HUGETLB): 0xffffffffffffffff - FAILED

Link: https://lkml.kernel.org/r/20250327114813.25980-1-liwang@redhat.com
Signed-off-by: Li Wang <liwang@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>