[libc] Add implementation of difftime function.

The difftime function computes the difference between two calendar
times: time1 - time0 as per as per 7.27.2.2 section in
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2478.pdf

.

  double difftime(time_t time1, time_t time0);

Tested:
Unit tests

Co-authored-by: Jeff Bailey <jeffbailey@google.com>

Reviewed By: jeffbailey

Differential Revision: https://reviews.llvm.org/D136631