[libc] Implement localtime

[zimirza]

localtime should be implemented for libc, which is required by #107285. Temporarily, I am using gmtime. Timezones need to be set up as well.

According to documentation, localtime is defined as the following:

struct tm *localtime(const time_t *timep);
struct tm *localtime_r(const time_t *timep, struct tm *result);
int localtime_s(struct tm *timep, time_t *timep);

[nickdesaulniers]

localtime_r is used in llvm, so that would be useful to have, too.

/llvm-project-main/llvm/lib/Support/Chrono.cpp:30:21: error: no member named 'localtime_r' in the global namespace
   30 |   struct tm *LT = ::localtime_r(&OurTime, &Storage);
      |                   ~~^

[zimirza]

Thank you for the feedback. I will look into if it can find localtime_r in #110363. I have an implementation there.