Several key targets unconditionally depend on `OSUtil.osutil` target,
causing errors when it is unnecessarily linked, or not available. This
PR fine-tuning the dependency on `OSUtil.osutil` to cleanly decouple
those targets, and gracefully skip targets that need `osutil`. Main
changes include:
* Make `LIBC_COPT_USE_C_ASSERT` to a cmake config, allowing
`LIBC_ASSERT` to use system's `assert` and not depending on `osutil`.
* Adjust cmake dependency for the following targets:
- libc.src.__support.libc_assert
- libc.src.__support.time.*
- libc.src.time.linux.*
- libc.src.unistd.*
- LibcTest
* Give an option for `TestLogger` to use system's `fprintf` instead of
`osutil`.
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