cast to signed char in ldb_cmd_test for ppc64le
cast to signed char in ldb_cmd_test for ppc64le

Summary:
char is unsigned on power by default causing this test to fail with the FF case. ppc64 return 255 while x86 returned -1. Casting works on both platforms.
Closes https://github.com/facebook/rocksdb/pull/1500

Differential Revision: D4308775

Pulled By: yiwu-arbug

fbshipit-source-id: db3e6e0

bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb