selftests/powerpc: Don't touch VMX/VSX on older CPUs

If we're running on a CPU without VMX/VSX then don't touch them. This
is fragile, the compiler could spill a VMX/VSX register and break the
test anyway. But in practice it seems to work, ie. the test runs to
completion on a system without VSX with this change.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819015727.1977134-8-mpe@ellerman.id.au

tools/testing/selftests/powerpc/benchmarks/context_switch.c

@@ -481,6 +481,12 @@ int main(int argc, char *argv[])
 	else
 		printf("futex");
 
+	if (!have_hwcap(PPC_FEATURE_HAS_ALTIVEC))
+		touch_altivec = 0;
+
+	if (!have_hwcap(PPC_FEATURE_HAS_VSX))
+		touch_vector = 0;
+
 	printf(" on cpus %d/%d touching FP:%s altivec:%s vector:%s vdso:%s\n",
 	       cpu1, cpu2, touch_fp ?  "yes" : "no", touch_altivec ? "yes" : "no",
 	       touch_vector ? "yes" : "no", touch_vdso ? "yes" : "no");