[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 f

[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 1 target `TARGET_NAME.__NO_FLAG_NAME` that
depends on `DEP.__NO_FLAG_NAME` if `DEP` is in its DEPENDS list and `DEP`
has `FLAG_NAME` in its `FLAGS` property.

To show all the targets generated, pass SHOW_INTERMEDIATE_OBJECTS=ON to cmake.
To show all the targets' dependency and flags, pass
`SHOW_INTERMEDIATE_OBJECTS=DEPS` to cmake.

To completely disable a flag FLAG_NAME expansion, set the variable
`SKIP_FLAG_EXPANSION_FLAG_NAME=TRUE`.

Reviewed By: michaelrj, sivachandra

Differential Revision: https://reviews.llvm.org/D125174

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