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>
13 files changed