leveldb: Fix compilation warnings in port_posix_sse.cc on x86 (32-bit).

LE_LOAD64 is only used when _mm_crc32_u64 is available, on 64-bit x86 processors.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148906169

port/port_posix_sse.cc

@@ -37,13 +37,17 @@ static inline uint32_t LE_LOAD32(const uint8_t *p) {
   return word;
 }
 
+#if defined(_M_X64) || defined(__x86_64__)  // LE_LOAD64 is only used on x64.
+
 // Used to fetch a naturally-aligned 64-bit word in little endian byte-order
 static inline uint64_t LE_LOAD64(const uint8_t *p) {
   uint64_t dword;
   memcpy(&dword, p, sizeof(dword));
   return dword;
 }
 
+#endif  // defined(_M_X64) || defined(__x86_64__)
+
 static inline bool HaveSSE42() {
 #if defined(_MSC_VER)
   int cpu_info[4];