Default one to rocksdb:x64-windows
Default one to rocksdb:x64-windows

Summary:
The default one will try to install rocksdb:x86-windows, which would lead to failing of the build at the last step (CMake Error, Rocksdb only supports x64). Because it will try to install a serials of x86 version package, and those cannot proceed to rocksdb:x86-windows building. By using rocksdb:x64-windows, we can make sure to install x64 version.
Tested on Win10 x64.
Closes https://github.com/facebook/rocksdb/pull/2941

Differential Revision: D5937139

Pulled By: sagar0

fbshipit-source-id: 15637fe23df59326a0e607bd4d5c48733e20bae3

SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb