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

This reverts commit 6342aa3.

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 <[email protected]>

lib/CMakeLists.txt

@@ -4,10 +4,10 @@
 add_compile_options($<TARGET_PROPERTY:compiler,warning_shadow_variables>)
 
 add_subdirectory(crc)
-if(NOT CONFIG_NATIVE_LIBC)
+if(NOT CONFIG_EXTERNAL_LIBC)
 add_subdirectory(libc)
-endif()
 add_subdirectory(posix)
+endif()
 add_subdirectory_ifdef(CONFIG_CPP                  cpp)
 add_subdirectory(hash)
 add_subdirectory(heap)