cmake: add option WITH_CORE_TOOLS to exclude tools except ldb and sst_dump (#6506)

cmake: add option WITH_CORE_TOOLS to exclude tools except ldb and sst_dump (#6506)

Summary:
ldb and sst_dump are most important tools and they don't dependend on gflags. In cmake, we don't have an way to only build these two tools and exclude other tools. This is inconvenient if the environment has a problem with gflags. Add such an option WITH_CORE_TOOLS.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6506

Test Plan: cmake and build with WITH_TOOLS and without.

Differential Revision: D20473029

fbshipit-source-id: 3d730fd14bbae6eeeae7f9cc9aec50a4e488ad72