Refactor FP_FAST_* into bits/fp-fast.h.

Refactor FP_FAST_* into bits/fp-fast.h.

Continuing the refactoring of bits/mathdef.h, this patch moves the
FP_FAST_* definitions into a new bits/fp-fast.h header.  Currently
this is only for FP_FAST_FMA*, but in future it would be the
appropriate place for the FP_FAST_* macros from TS 18661-1 as well.

The generic bits/mathdef.h header defines these macros based on
whether the compiler defines __FP_FAST_*.  Most architecture-specific
headers, however, fail to do so, meaning that if the architecture (or
some particular processors) does in fact have fused operations, and
GCC knows to use them inline, the FP_FAST_* macros will still not be
defined.

By refactoring, this patch causes the generic version (based on
__FP_FAST_*) to be used in more cases, and so the macro definitions to
be more accurate.  Architectures that already defined some or all of
these macros other than based on the predefines have their own
versions of fp-fast.h, which are arranged so they define FP_FAST_* if
either the architecture-specific conditions are true or __FP_FAST_*
are defined.

After this refactoring, various bits/mathdef.h headers for
architectures with long double = double are semantically identical to
the generic version.  The patch removes those headers that are
redundant.  (In fact two of the four removed were already redundant
before this patch because they did use __FP_FAST_*.)

Tested for x86_64 and x86, and compilation-only with
build-many-glibcs.py.

	* bits/fp-fast.h: New file.
	* sysdeps/aarch64/bits/fp-fast.h: Likewise.
	* sysdeps/powerpc/bits/fp-fast.h: Likewise.
	* math/Makefile (headers): Add bits/fp-fast.h.
	* math/math.h: Include <bits/fp-fast.h>.
	* bits/mathdef.h (FP_FAST_FMA): Remove.
	(FP_FAST_FMAF): Likewise.
	(FP_FAST_FMAL): Likewise.
	* sysdeps/aarch64/bits/mathdef.h (FP_FAST_FMA): Likewise.
	(FP_FAST_FMAF): Likewise.
	* sysdeps/powerpc/bits/mathdef.h (FP_FAST_FMA): Likewise.
	(FP_FAST_FMAF): Likewise.
	* sysdeps/x86/bits/mathdef.h (FP_FAST_FMA): Likewise.
	(FP_FAST_FMAF): Likewise.
	(FP_FAST_FMAL): Likewise.
	* sysdeps/arm/bits/mathdef.h: Remove file.
	* sysdeps/hppa/fpu/bits/mathdef.h: Likewise.
	* sysdeps/sh/sh4/bits/mathdef.h: Likewise.
	* sysdeps/tile/bits/mathdef.h: Likewise.