Add arch-specific configuration for C11 atomics support.

This sets __HAVE_64B_ATOMICS if provided.  It also sets
USE_ATOMIC_COMPILER_BUILTINS to true if the existing atomic ops use the
__atomic* builtins (aarch64, mips partially) or if this has been
tested (x86_64); otherwise, this is set to false so that C11 atomics will
be based on the existing atomic operations.
Add atomic operations similar to those provided by C11.
Add tests for C11-like atomic operations.
Use C11 atomics in pthread_once.
NPTL: Refactor scheduler setup in pthread_create.
NPTL: Conditionalize asynchronous cancellation support on [SIGCANCEL].
NPTL: Use __libc_fatal in unwind.c.