Fix math_private.h multiple include guards.

Fix math_private.h multiple include guards.

Various math_private.h headers are guarded by "#ifndef
_MATH_PRIVATE_H", but never define the macro.  Nothing else defines
the macro either (the generic math_private.h that they include defines
a different macro, _MATH_PRIVATE_H_), so those guards are ineffective.

With the recent inclusion of s_sin.c in s_sincos.c, this breaks the
build for MIPS, since the build of s_sincos.c ends up including
<math_private.h> twice and the MIPS version defines inline functions
such as libc_feholdexcept_mips, without a separate fenv_private.h
header with its own guards such as some architectures have.

This patch fixes all the problem headers to use architecture-specific
guard macro names, and to define those macros in the headers they
guard, just as some architectures already do.

Tested for x86 (testsuite, and that installed shared libraries are
unchanged by the patch), and for mips64 (that it fixes the build).

	* sysdeps/arm/math_private.h [!_MATH_PRIVATE_H]: Change guard to
	[!ARM_MATH_PRIVATE_H].
	[!ARM_MATH_PRIVATE_H] (ARM_MATH_PRIVATE_H): Define macro.
	* sysdeps/hppa/math_private.h [!_MATH_PRIVATE_H]: Change guard to
	[!HPPA_MATH_PRIVATE_H].
	[!HPPA_MATH_PRIVATE_H] (HPPA_MATH_PRIVATE_H): Define macro.
	* sysdeps/i386/fpu/math_private.h [!_MATH_PRIVATE_H]: Change guard
	to [!I386_MATH_PRIVATE_H].
	[!I386_MATH_PRIVATE_H] (I386_MATH_PRIVATE_H): Define macro.
	* sysdeps/m68k/m680x0/fpu/math_private.h [!_MATH_PRIVATE_H]:
	Change guard to [!M68K_MATH_PRIVATE_H].
	[!M68K_MATH_PRIVATE_H] (M68K_MATH_PRIVATE_H): Define macro.
	* sysdeps/microblaze/math_private.h [!_MATH_PRIVATE_H]: Change
	guard to [!MICROBLAZE_MATH_PRIVATE_H].
	[!MICROBLAZE_MATH_PRIVATE_H] (MICROBLAZE_MATH_PRIVATE_H): Define
	macro.
	* sysdeps/mips/math_private.h [!_MATH_PRIVATE_H]: Change guard to
	[!MIPS_MATH_PRIVATE_H].
	[!MIPS_MATH_PRIVATE_H] (MIPS_MATH_PRIVATE_H): Define macro.
	* sysdeps/nios2/math_private.h [!_MATH_PRIVATE_H]: Change guard to
	[!NIO2_MATH_PRIVATE_H].
	[!NIO2_MATH_PRIVATE_H] (NIO2_MATH_PRIVATE_H): Define macro.
	* sysdeps/tile/math_private.h [!_MATH_PRIVATE_H]: Change guard to
	[!TILE_MATH_PRIVATE_H].
	[!TILE_MATH_PRIVATE_H] (TILE_MATH_PRIVATE_H): Define macro.