lib: posix: Use "posix_subsys" as the CMake lib for the subsystem.
lib: posix: Use "posix_subsys" as the CMake lib for the subsystem.

Historically, it used to be "PTHREAD", which is no longer true, as
POSIX subsys offers much more functionality than just Pthreads. Use
detailed name, like "posix_subsys", to avoid possible confusion with
ARCH_POSIX-related matters.

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

Vudentz/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr