Revert "lib: support linking to POSIX API with 3rd-party libc"

This reverts commit 6342aa3c.

This commit should never have been merged.
Apart from the fact that this change was rejected in previous
review,
this change is wrong, for 2 reasons:
1. The POSIX_API (POSIX compatibility shim) cannot be
built if the host libC is used.
2. The Zephyr libC CMake files were guarded so they
would not be dragged when CONFIG_EXTERNAL_LIBC was selected.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>