Summary:
Slightly changes the format of generated BUCK files for Facebook consumption. Generated targets end up looking like this:
```
cpp_library(
    name = "rocksdb_tools_lib",
    srcs = [
        "tools/db_bench_tool.cc",
        "tools/trace_analyzer_tool.cc",
        "util/testutil.cc",
    ],
    auto_headers = AutoHeaders.RECURSIVE_GLOB,
    arch_preprocessor_flags = rocksdb_arch_preprocessor_flags,
    compiler_flags = rocksdb_compiler_flags,
    preprocessor_flags = rocksdb_preprocessor_flags,
    deps = [":rocksdb_lib"],
    external_deps = rocksdb_external_deps,
)
```
Instead of
```
cpp_library(
    name = "rocksdb_tools_lib",
    srcs = [
        "tools/db_bench_tool.cc",
        "tools/trace_analyzer_tool.cc",
        "util/testutil.cc",
    ],
    headers = AutoHeaders.RECURSIVE_GLOB,
    arch_preprocessor_flags = rocksdb_arch_preprocessor_flags,
    compiler_flags = rocksdb_compiler_flags,
    preprocessor_flags = rocksdb_preprocessor_flags,
    deps = [":rocksdb_lib"],
    external_deps = rocksdb_external_deps,
)
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4624

Reviewed By: riversand963

Differential Revision: D12906711

Pulled By: philipjameson

fbshipit-source-id: 32ab64a3390cdcf2c4043ff77517ac1ad58a5e2b