libc: common: implement multiple time functions

Implemented the following:
- `asctime_r()`
- `asctime()`
- `localtime()`
- `localtime_r()`
- `ctime()`
- `ctime_r()`

Specifically:
- the implementation of `localtime()` & `localtime_r()` simply
  wraps around the gmtime() & gmtime_r() functions, the
  results are always expressed as UTC.
- `ctime()` is equivalent to `asctime(localtime(clock))`, it
  inherits the limitation of `localtime()` as well, which only
  supports UTC results currently.

Added tests for these newly implemented functions.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>

doc/services/portability/posix/option_groups/index.rst

@@ -981,8 +981,8 @@ Enable this option with :kconfig:option:`CONFIG_POSIX_THREAD_SAFE_FUNCTIONS`.
     :header: API, Supported
     :widths: 50,10
 
-    asctime_r(),
-    ctime_r(),
+    asctime_r(), yes
+    ctime_r(), yes (UTC timezone only)
     flockfile(),
     ftrylockfile(),
     funlockfile(),
@@ -993,7 +993,7 @@ Enable this option with :kconfig:option:`CONFIG_POSIX_THREAD_SAFE_FUNCTIONS`.
     getpwnam_r(),yes :ref:`†<posix_undefined_behaviour>`
     getpwuid_r(),yes :ref:`†<posix_undefined_behaviour>`
     gmtime_r(), yes
-    localtime_r(),
+    localtime_r(), yes (UTC timezone only)
     putc_unlocked(),
     putchar_unlocked(),
     rand_r(), yes