lib: libc: Move newlib libc configs to lib/libc/newlib/Kconfig

This commit relocates the newlib libc configurations under the top-
level libc Kconfig to a new Kconfig under `lib/libc/newlib` for
improved organisation of the libc configurations.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>