Added support for atomic floating point operations.

The support includes:

- The standard fetch_add/fetch_sub operations.
- Extra operations: (fetch_/opaque_)negate, (opaque_)add/sub.
- Extra capability macros: BOOST_ATOMIC_FLOAT/DOUBLE/LONG_DOUBLE_LOCK_FREE.

The atomic operations are currently implemented on top of the integer-based
backends and thus are mostly CAS-based. The CAS operations perform binary
comparisons, and as such have different behavior wrt. special FP values like
NaN and signed zero than normal C++.

The support for floating point types is optional and can be disabled by
defining BOOST_ATOMIC_NO_FLOATING_POINT. This can be useful if on a certain
platform parameters of the floating point types cannot be deduced from the
compiler-defined or system macros (in which case the compilation fails).

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0020r6.html

test/native_api.cpp

@@ -45,6 +45,13 @@ int main(int, char *[])
     test_constexpr_ctor<long>();
     test_constexpr_ctor<int*>();
 
+    test_floating_point_api<float>();
+    test_floating_point_api<double>();
+    test_floating_point_api<long double>();
+#if defined(BOOST_HAS_FLOAT128)
+    test_floating_point_api<boost::float128_type>();
+#endif
+
     test_pointer_api<int>();
 
     test_enum_api();