Split fenv_private.h out of math_private.h more consistently.

Split fenv_private.h out of math_private.h more consistently.

On some architectures, the parts of math_private.h relating to the
floating-point environment are in a separate file fenv_private.h
included from math_private.h.  As this is purely an
architecture-specific convention used by several architectures,
however, all such architectures still need their own math_private.h,
even if it has nothing to do beyond #include <fenv_private.h> and
peculiarity of including the i386 file directly instead of having a
shared file in sysdeps/x86.

This patch makes the fenv_private.h name an architecture-independent
convention in glibc.  The include of fenv_private.h from
math_private.h becomes architecture-independent (until callers are
updated to include fenv_private.h directly so the include from
math_private.h is no longer needed).  Some architecture math_private.h
headers are removed if no longer needed, or renamed to fenv_private.h
if all they define belongs in that header; architecture fenv_private.h
headers now do require #include_next <fenv_private.h>.  The i386
fenv_private.h file moves to sysdeps/x86/fpu/ to reflect how it is
actually shared with x86_64.  The generic math_private.h gets a new
include of <stdbool.h>, as needed for bool in some prototypes in that
header (previously that was indirectly included via include/fenv.h,
which now only gets included too late in math_private.h, after those
prototypes).

Tested for x86_64 and x86, and tested with build-many-glibcs.py that
installed stripped shared libraries are unchanged by the patch.

	* sysdeps/aarch64/fpu/fenv_private.h: New file.  Based on ....
	* sysdeps/aarch64/fpu/math_private.h: ... this file.  All contents
	moved to fenv_private.h except for ...
	(TOINT_INTRINSICS): Kept in math_private.h.
	(roundtoint): Likewise.
	(converttoint): Likewise.
	* sysdeps/arm/fenv_private.h: Change multiple-include guard to
	[ARM_FENV_PRIVATE_H].  Include next <fenv_private.h>.
	* sysdeps/arm/math_private.h: Remove.
	* sysdeps/generic/fenv_private.h: New file.  Contents moved from
	....
	* sysdeps/generic/math_private.h: ... this file.  Include
	<stdbool.h>.  Do not include <fenv.h> or <get-rounding-mode.h>.
	Include <fenv_private.h>.  Remove functions and macros moved to
	fenv_private.h.
	* sysdeps/i386/fpu/math_private.h: Remove.
	* sysdeps/mips/math_private.h: Move to ....
	* sysdeps/mips/fpu/fenv_private.h: ... here.  Change
	multiple-include guard to [MIPS_FENV_PRIVATE_H].  Remove
	[__mips_hard_float] conditional.  Include next <fenv_private.h>.
	* sysdeps/powerpc/fpu/fenv_private.h: Change multiple-include
	guard to [POWERPC_FENV_PRIVATE_H].  Include next <fenv_private.h>.
	* sysdeps/powerpc/fpu/math_private.h: Do not include
	<fenv_private.h>.
	* sysdeps/riscv/rvf/math_private.h: Move to ....
	* sysdeps/riscv/rvf/fenv_private.h: ... here.  Change
	multiple-include guard to [RISCV_FENV_PRIVATE_H].  Include next
	<fenv_private.h>.
	* sysdeps/sparc/fpu/fenv_private.h: Change multiple-include guard
	to [SPARC_FENV_PRIVATE_H].  Include next <fenv_private.h>.
	* sysdeps/sparc/fpu/math_private.h: Remove.
	* sysdeps/i386/fpu/fenv_private.h: Move to ....
	* sysdeps/x86/fpu/fenv_private.h: ... here.  Change
	multiple-include guard to [X86_FENV_PRIVATE_H].  Include next
	<fenv_private.h>.
	* sysdeps/x86_64/fpu/math_private.h: Do not include
	<sysdeps/i386/fpu/fenv_private.h>.