[libc] Implement the strftime and strftime_l entrypoints

[jhuber6]

These functions are required to build the LLVM C++ library on top of the LLVM C library. We currently only support the default (POSIX) locale, so the implementation should ideally be more straightforward ignoring all the details in langinfo.h and locale.h.

[nickdesaulniers]

I was able to turn off parts of libc++ in order to build it, but then building llvm on top of that failed, due to:

/llvm-project-main/llvm/lib/Support/Chrono.cpp:64:3: error: use of undeclared identifier 'strftime'.
   64 |   strftime(Buffer, sizeof(Buffer), "%Y-%m-%d %H:%M:%S", &LT);
      |   ^
/llvm-project-main/llvm/lib/Support/Chrono.cpp:105:16: error: use of undeclared identifier
 'strftime'
  105 |   size_t Len = strftime(Buffer, sizeof(Buffer), Format.c_str(), &LT);
      |                ^