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 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).

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Properly parenthesize the __tg_is_complex() macro by moving a closing
paren to where it belongs.

MFC after: 7 days

Add a workaround to recognise I/_Complex_I as complex arguments. Although
the GCC manual claims that the expression 1.0fi has type float _Complex,
__builtin_types_compatible_p(float _Complex, __typeof__(1.0fi))) yields 0.

Use the keyword '_Complex' rather than the macro 'complex' since
applications are allowed to undefine the latter.

Embarrassing typo: s/nextbyint/nearbyint/

Implement C99's standard header <tgmath.h>. It provides type-generic macros
for the <math.h> and <complex.h> functions that have float, double and long
double implementations. Such type-generic macros expand to an actual
function, depending on the types of the macro arguments, eg. if <tgmath.h>
is included, the invocation cos(1.0f) calls the function cosf().