libroot: Replace the old localtime fallback backend with musl code.

This allows for the deletion of quite a lot of ugly C89 code.

libroot: Replace asctime[_r] with musl's.

This broke anyway in hrev56361.

Change-Id: I1e7e02b27d6fee4845c81cf6f229bca0048a0e61

libroot: Replace strftime/strptime/wcsftime implementations with musl's.

Now that we have locale_t, we can use the musl versions of these functions.
This also fixes a licensing issue: the strptime implementation had an
advertising clause (although in upstream *BSD it was removed, so we
likely could have managed to remove it anyway.)