selftests/powerpc: spectre_v2 test must be built 64-bit

The spectre_v2 test must be built 64-bit, it includes hand-written asm
that is 64-bit only, and segfaults if built 32-bit.

Fixes: c790c3d2 ("selftests/powerpc: Add a test of spectre_v2 mitigations")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191120023924.13130-1-mpe@ellerman.id.au