MFC r241077:

- Simplify the implementation of atomic_compare_exchange_strong_explicit.
- Evaluate the memory order argument in atomic_fetch_*_explicit macros.
- Implement atomic_store_explicit using atomic_exchange_explicit instead
  of a plain assignment.

MFC r241190:

Define clang feature test macro __has_extension. It's used in stdatomic.h.

MFC r254497:

Change the return type of the fallback implementation of the
atomic_compare_exchange_* macros in stdatomic.h to _Bool.
Provide a prototype for yyparse() so that "-Werror" along with stringent
-W* warnings can be used on byacc's output.
Fix an integer overflow in computing the size of a temporary buffer
can result in a buffer which is too small for the requested
operation.

Security:	CVE-2013-3077
Security:	FreeBSD-SA-13:09.ip_multicast
Merge remote-tracking branch 'upstream-github/stable/9' into stable/9