util: only allow _BitScanReverse64 on 64-bit cpus

While the documentation for _BitScanReverse64 on MSDN says that it's
available on ARM, this isn't true. It's only available on ARM64. So
let's match reality.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Acked-by: Matt Turner <mattst88@gmail.com>
22 jobs for !1694 with msvc-sse-fixes in 117 minutes and 43 seconds (queued for 40 seconds)
detached
Status Name Job ID Coverage
  Containers Build
passed debian #560427

00:00:18

 
  Build+Test
passed meson-arm64 #560436

00:02:13

passed meson-armhf #560435

00:03:23

passed meson-clang #560430

00:09:43

passed meson-clover #560433

00:10:46

passed meson-i386 #560437

00:06:03

passed meson-main #560428

00:09:25

passed meson-swr-glvnd #560429

00:05:37

passed meson-vulkan #560434

00:04:50

passed scons-llvm #560439

00:07:05

passed scons-nollvm #560438

00:05:05

passed scons-swr #560431

00:06:40

passed scons-win64 #560432

00:06:57

 
  Test
passed test-llvmpipe-gles2 1/4 #560440

00:04:46

passed test-llvmpipe-gles2 2/4 #560441

00:05:53

passed test-llvmpipe-gles2 3/4 #560442

00:09:48

passed test-llvmpipe-gles2 4/4 #560443

00:05:42

passed test-softpipe-gles2 1/4 #560444

00:01:47

passed test-softpipe-gles2 2/4 #560445

00:02:26

passed test-softpipe-gles2 3/4 #560446

00:02:10

passed test-softpipe-gles2 4/4 #560447

00:02:38

passed test-softpipe-gles3-limited #560448

00:04:17