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: c790c3d ("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

tools/testing/selftests/powerpc/security/Makefile

@@ -8,4 +8,6 @@ CFLAGS += -I../../../../../usr/include
 include ../../lib.mk
 
 $(TEST_GEN_PROGS): ../harness.c ../utils.c
+
+$(OUTPUT)/spectre_v2: CFLAGS += -m64
 $(OUTPUT)/spectre_v2: ../pmu/event.c branch_loops.S