kselftest/arm64: Only enumerate power of two VLs in syscall-abi

As documented in issue C215 in the known issues list for DDI0487I.a [1] Arm
will be making a retroactive change to SVE to remove the possibility of
selecting non power of two vector lengths. This has no impact on existing
physical implementations but most virtual implementations have implemented
the full range of permissible vector lengths.

Since virtual implementations are noticeably slow in general and the larger
vector lengths amplify the issue there's a useful improvement in runtime
from only covering the vector lengths that will exist in practical systems,
adjust our enumeration accordingly. We have other tests that aim to cover
the enumeration interfaces.

For symmetry we apply the same change to the eumeration for SME vector
lengths, though the power of two restriction was already present for SME
so there is no impact on the set of vector lengths tested.

[1] https://developer.arm.com/documentation/102105/ia-00/Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221223-arm64-syscall-abi-sme-only-v1-4-4fabfbd62087@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/abi/syscall-abi.c

@@ -444,7 +444,7 @@ void sve_count_vls(void)
 	/*
 	 * Enumerate up to SVE_VQ_MAX vector lengths
 	 */
-	for (vq = SVE_VQ_MAX; vq > 0; --vq) {
+	for (vq = SVE_VQ_MAX; vq > 0; vq /= 2) {
 		vl = prctl(PR_SVE_SET_VL, vq * 16);
 		if (vl == -1)
 			ksft_exit_fail_msg("PR_SVE_SET_VL failed: %s (%d)\n",
@@ -470,7 +470,7 @@ void sme_count_vls(void)
 	/*
 	 * Enumerate up to SVE_VQ_MAX vector lengths
 	 */
-	for (vq = SVE_VQ_MAX; vq > 0; --vq) {
+	for (vq = SVE_VQ_MAX; vq > 0; vq /= 2) {
 		vl = prctl(PR_SME_SET_VL, vq * 16);
 		if (vl == -1)
 			ksft_exit_fail_msg("PR_SME_SET_VL failed: %s (%d)\n",