Cleanup the Arm64 CRC32 unused warning (#5565)

Summary:
When 'HAVE_ARM64_CRC' is set, the blew methods:

- bool rocksdb::crc32c::isSSE42()
- bool rocksdb::crc32c::isPCLMULQDQ()

are defined but not used, the unused-function is raised
when do rocksdb build.

This patch try to cleanup these warnings by add ifndef,
if it build under the HAVE_ARM64_CRC, we will not define
`isSSE42` and `isPCLMULQDQ`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5565

Differential Revision: D16233654

fbshipit-source-id: c32a9dda7465dbf65f9ccafef159124db92cdffd

util/crc32c.cc

@@ -398,6 +398,8 @@ uint32_t ExtendImpl(uint32_t crc, const char* buf, size_t size) {
   return static_cast<uint32_t>(l ^ 0xffffffffu);
 }
 
+// Detect if ARM64 CRC or not.
+#ifndef HAVE_ARM64_CRC
 // Detect if SS42 or not.
 #ifndef HAVE_POWER8
 
@@ -436,6 +438,7 @@ static bool isPCLMULQDQ() {
 }
 
 #endif  // HAVE_POWER8
+#endif  // HAVE_ARM64_CRC
 
 typedef uint32_t (*Function)(uint32_t, const char*, size_t);