[libc] Implement FLAGS option for generating all combinations for targets.

Add FLAGS option for add_header_library, add_object_library,
add_entrypoint_object, and add_libc_unittest.

In general, a flag is a string provided for supported functions under the
multi-valued option `FLAGS`.  It should be one of the following forms:
  FLAG_NAME
  FLAG_NAME__NO
  FLAG_NAME__ONLY
A target will inherit all the flags of its upstream dependency.

When we create a target `TARGET_NAME` with a flag using (add_header_library,
add_object_library, ...), its behavior will depend on the flag form as follow:
- FLAG_NAME: The following 2 targets will be generated:
    `TARGET_NAME` that has `FLAG_NAME` in its `FLAGS` property.
    `TARGET_NAME.__NO_FLAG_NAME` that depends on `DEP.__NO_FLAG_NAME` if
       `TARGET_NAME` depends on `DEP` and `DEP` has `FLAG_NAME` in its `FLAGS`
       property.
- FLAG_NAME__ONLY: Only generate 1 target `TARGET_NAME` that has `FLAG_NAME`
    in its `FLAGS` property.
- FLAG_NAME__NO: Only generate...