Simplify C99 isgreater macros

Simplify C99 isgreater macros

Simplify the C99 isgreater macros.  Although some support was added
in GCC 2.97, not all targets added support until GCC 3.1.  Therefore
only use the builtins in math.h from GCC 3.1 onwards, and defer to
generic macros otherwise.  Improve the generic isunordered macro
to use compares rather than call fpclassify twice - this is not only
faster but also correct for signaling NaNs.

	* math/math.h: Improve handling of C99 isgreater macros.
	* sysdeps/alpha/fpu/bits/mathinline.h: Remove isgreater macros.
	* sysdeps/m68k/m680x0/fpu/bits/mathinline.h: Likewise.
	* sysdeps/powerpc/bits/mathinline.h: Likewise.
	* sysdeps/sparc/fpu/bits/mathinline.h: Likewise.
	* sysdeps/x86/fpu/bits/mathinline.h: Likewise.