selftests/powerpc: Add helper to exit on failure

This adds a helper similar to FAIL_IF() which lets a
program exit with code 1 (to indicate failure) when
the given condition is true.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/dac282d5c2e96e7816dc522e4e20d56d7c79c898.1595821792.git.sandipan@linux.ibm.com