ORC-1455: [C++] Fix build with unused macro in CpuInfoUtil.cc (#1552)

### What changes were proposed in this pull request?
Move UNUSED macro under the definition of CPUINFO_ARCH_X86 in order to avoid unused macro error.

### Why are the changes needed?
Without this patch, old C++(Clang 13 and older) build fails on MacOS and other non-x86 machines.

### How was this patch tested?
It builds successfully and passes all tests on my laptop.

c++/src/CpuInfoUtil.cc

@@ -55,16 +55,15 @@
 
 #if defined(__i386) || defined(_M_IX86) || defined(__x86_64__) || defined(_M_X64)
 #define CPUINFO_ARCH_X86
+#ifndef ORC_HAVE_RUNTIME_AVX512
+#define UNUSED(x) (void)(x)
+#endif
 #elif defined(_M_ARM64) || defined(__aarch64__) || defined(__arm64__)
 #define CPUINFO_ARCH_ARM
 #elif defined(__PPC64__) || defined(__PPC64LE__) || defined(__ppc64__) || defined(__powerpc64__)
 #define CPUINFO_ARCH_PPC
 #endif
 
-#ifndef ORC_HAVE_RUNTIME_AVX512
-#define UNUSED(x) (void)(x)
-#endif
-
 namespace orc {
 
   namespace {