selftests/powerpc: Always build the tm-poison test 64-bit

The tm-poison test includes inline asm which is 64-bit only, so the
test must be built 64-bit in order to work.

Otherwise it fails, eg:
  # file tm-poison
  tm-poison: ELF 32-bit MSB executable, PowerPC or cisco 4500, version 1 (SYSV) ...
  # ./tm-poison
  test: tm_poison_test
  Unknown value 0x1fff71150 leaked into f31!
  Unknown value 0x1fff710c0 leaked into vr31!
  failure: tm_poison_test

Fixes: a003365c

 ("powerpc/tm: Add tm-poison test")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200403095656.3772005-1-mpe@ellerman.id.au