[libc] dead/broken entrypoints

[SchrodingerZhu]

After #101797, I still get the following test demanding some non-existing entrypoints:

[cmake] -- Skipping unittest libc.test.src.stdio.scanf_core.converter_test as it has missing deps: libc.src.stdio.scanf_core.string_reader.
[cmake] -- Skipping unittest libc.test.src.wchar.btowc_test.__unit__ as it has missing deps: libc.src.wchar.btowc.
[cmake] -- Skipping hermetic test libc.test.src.wchar.btowc_test.__hermetic__ as it has missing deps: libc.src.wchar.btowc.

[SchrodingerZhu]

cc @michaelrj-google since this is related to wchar and stdio.

[michaelrj-google]

iirc btowc is broken because I forgot to finish setting up the cmake, and scanf_core.converter_test just needs a minor rewrite to handle the new scanf writer. Both small fixes that I just haven't gotten around to