Refactor some libm type-generic macros.

Refactor some libm type-generic macros.

This patch refactors some type-generic libm macros, in both math.h and
math_private.h, to be based on a common __MATH_TG macro rather than
all replicating similar logic to choose a function to call based on
the type of the argument.

This should serve to illustrate what I think float128 support for such
macros should look like: common macros such as __MATH_TG may need
different definitions depending on whether float128 is supported in
glibc, so that the individual macros themselves do not need
conditionals on float128 support.

Tested for x86_64, x86, mips64 and powerpc.

	* math/math.h (__MATH_TG): New macro.
	[__USE_ISOC99] (fpclassify): Define using __MATH_TG.
	[__USE_ISOC99] (signbit): Likewise.
	[__USE_ISOC99] (isfinite): Likewise.
	[__USE_ISOC99] (isnan): Likewise.
	[__USE_ISOC99] (isinf): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT)] (issignaling): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT)] (__MATH_EVAL_FMT2): New macro.
	[__GLIBC_USE (IEC_60559_BFP_EXT)] (iseqsig): Define using
	__MATH_TG and __MATH_EVAL_FMT2.
	* sysdeps/generic/math_private.h (fabs_tg): Define using
	__MATH_TG.
	* sysdeps/ieee754/ldbl-128ibm/bits/iscanonical.h
	[!__NO_LONG_DOUBLE_MATH] (__iscanonicalf): New macro.
	[!__NO_LONG_DOUBLE_MATH] (__iscanonical): Likewise.
	[!__NO_LONG_DOUBLE_MATH] (iscanonical): Define using __MATH_TG.
	* sysdeps/ieee754/ldbl-96/bits/iscanonical.h (__iscanonicalf): New
	macro.
	(__iscanonical): Likewise.
	(iscanonical): Define using __MATH_TG.