Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r263998:

Rename __wchar_t so it no longer conflicts with __wchar_t from clang 3.4
-fms-extensions.

MFC r254465: Correct implementation of atomic_flag_test_and_set

(This is a direct commit to stable/9 as atomic_flag_test_and_set_explicit
is an inline function on HEAD and in a different file.)

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.

MFC r251192: Fix misspelling of structure field name.

MFC r240970:

- Make C11 atomic macros usable in expressions:
- Replace do-while statements with void expressions.
- Wrap __asm statements in statement expressions.
- Make the macros function-like:
- Evaluate all arguments exactly once.
- Make sure there's a sequence point between evaluation of the
arguments and the function body. Arguments should be evaluated
before any memory barriers.
- Fix use of __atomic_is_lock_free built-in. It requires the address
of an atomic variable as second argument. Use this built-in on clang
as well because clang's __c11_atomic_is_lock_free only takes the size
of the variable into account.
- In atomic_exchange_explicit put the barrier before instead of after
the __sync_lock_test_and_set call.

MFC r239960:

Properly enable Clang-style atomics when available.

In addition to testing against cxx_atomic, we must check c_atomic. The
former is only set when building C++ code. Also use __has_extension
instead of __has_feature. This allows us to use the atomics outside of
C11.

PR: threads/170073

MFC r228862, 228880, 228882, 228906, 228977, 229332, 229575, 229591, 229704,
229716, 230062, 230225, 230267, 234958

This brings stdatomic.h into -STABLE and includes improvements to tgmath.h that
significantly decrease compile times on a C11 compiler (such as the clang in
the base system).

Initial implementation of stdatomic.h. Works (at least in the parts that were
tested) with clang and gcc, and more efficiently with clang+a big out-of-tree
diff that I need to commit soon (once it's been tidied and reviewed a bit).

Large portions by: ed
Reviewed by: ed
Approved by: dim (mentor)