Use C11 *_TRUE_MIN macros where applicable.

Use C11 *_TRUE_MIN macros where applicable.

C11 defines standard <float.h> macros *_TRUE_MIN for the least
positive subnormal value of a type.  Now that we build with
-std=gnu11, we can use these macros in glibc.  This patch replaces
previous uses of the GCC predefines __*_DENORM_MIN__ (used in
<float.h> to define *_TRUE_MIN), as well as *_DENORM_MIN references in
comments.

Tested for x86_64 and x86 (testsuite, and that installed shared
libraries are unchanged by the patch).  Also tested for powerpc that
installed stripped shared libraries are unchanged by the patch.

	* math/libm-test.inc (min_subnorm_value): Use LDBL_TRUE_MIN,
	DBL_TRUE_MIN and FLT_TRUE_MIN instead of __LDBL_DENORM_MIN__,
	__DBL_DENORM_MIN__ and __FLT_DENORM_MIN__.
	* sysdeps/ieee754/dbl-64/s_fma.c (__fma): Refer to DBL_TRUE_MIN
	instead of DBL_DENORM_MIN in comment.
	* sysdeps/ieee754/ldbl-128/s_fmal.c (__fmal): Refer to
	LDBL_TRUE_MIN instead of LDBL_DENORM_MIN in comment.
	* sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c: Include <float.h>.
	(__nextafterl): Use LDBL_TRUE_MIN instead of __LDBL_DENORM_MIN__.
	* sysdeps/ieee754/ldbl-96/s_fmal.c (__fmal): Refer to
	LDBL_TRUE_MIN instead of LDBL_DENORM_MIN in comment.