Fix assertion in PosixRandomAccessFile

Summary:
See https://github.com/facebook/rocksdb/issues/244#issuecomment-53372297
Also see this: https://github.com/facebook/rocksdb/blob/master/util/env_posix.cc#L1075

Test Plan: compiles

Reviewers: yhchiang, ljin, sdong

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22419

util/env_posix.cc

@@ -231,7 +231,7 @@ class PosixRandomAccessFile: public RandomAccessFile {
   PosixRandomAccessFile(const std::string& fname, int fd,
                         const EnvOptions& options)
       : filename_(fname), fd_(fd), use_os_buffer_(options.use_os_buffer) {
-    assert(!options.use_mmap_reads);
+    assert(!options.use_mmap_reads || sizeof(void*) < 8);
   }
   virtual ~PosixRandomAccessFile() { close(fd_); }