selftest/powerpc: Fix false failures for skipped tests

Tests under alignment subdirectory are skipped when executed on previous
generation hardware, but harness still marks them as failed.

  test: test_copy_unaligned
  tags: git_version:unknown
  [SKIP] Test skipped on line 26
  skip: test_copy_unaligned
  selftests: copy_unaligned [FAIL]

The MAGIC_SKIP_RETURN_VALUE value assigned to rc variable is retained till
the program exit which causes the test to be marked as failed.

This patch resets the value before returning to the main() routine.
With this patch the test o/p is as follows:

  test: test_copy_unaligned
  tags: git_version:unknown
  [SKIP] Test skipped on line 26
  skip: test_copy_unaligned
  selftests: copy_unaligned [PASS]

Signed-off-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>