Support for jthread and stop_token

Support for jthread and stop_token

        * include/Makefile.am: Add <stop_token> header.
        * include/Makefile.in: Regenerate.
        * include/std/condition_variable: Add overloads for stop_token support
        to condition_variable_any.
        * include/std/stop_token: New file.
        * include/std/thread: Add jthread type.
        * include/std/version (__cpp_lib_jthread): New value.
        * testsuite/30_threads/condition_variable_any/stop_token/1.cc: New test.
        * testsuite/30_threads/condition_variable_any/stop_token/2.cc: New test.
        * testsuite/30_threads/condition_variable_any/stop_token/wait_on.cc: New test.
        * testsuite/30_threads/jthread/1.cc: New test.
        * testsuite/30_threads/jthread/2.cc: New test.
        * testsuite/30_threads/jthread/jthread.cc: New test.
        * testsuite/30_threads/stop_token/1.cc: New test.
        * testsuite/30_threads/stop_token/2.cc: New test.
        * testsuite/30_threads/stop_token/stop_token.cc: New test.

From-SVN: r278274