Support bits/floatn.h inclusion from .S files.

Support bits/floatn.h inclusion from .S files.

Further _FloatN / _FloatNx type alias support will involve making
architecture-specific .S files use the common macros for libm function
aliases.  Making them use those macros will also serve to simplify
existing code for aliases / symbol versions in various cases, similar
to such simplifications for ldbl-opt code.

The libm-alias-*.h files sometimes need to include <bits/floatn.h> to
determine which aliases they should define.  At present, this does not
work for inclusion from .S files because <bits/floatn.h> can define
typedefs for old compilers.  This patch changes all the
<bits/floatn.h> and <bits/floatn-common.h> headers to include
__ASSEMBLER__ conditionals.  Those conditionals disable everything
related to C syntax in the __ASSEMBLER__ case, not just the problem
typedefs, as that seemed cleanest.  The __HAVE_* definitions remain in
the __ASSEMBLER__ case, as those provide information that is required
to define the correct set of aliases.

Tested with build-many-glibcs.py for a representative set of
configurations (x86_64-linux-gnu i686-linux-gnu ia64-linux-gnu
powerpc64le-linux-gnu mips64-linux-gnu-n64 sparc64-linux-gnu) with GCC
6.  Also tested with GCC 6 for i686-linux-gnu in conjunction with
changes to use alias macros in .S files.

	* bits/floatn-common.h [!__ASSEMBLER]: Disable everything related
	to C syntax instead of availability and properties of types.
	* bits/floatn.h [!__ASSEMBLER]: Likewise.
	* sysdeps/ia64/bits/floatn.h [!__ASSEMBLER]: Likewise.
	* sysdeps/ieee754/ldbl-128/bits/floatn.h [!__ASSEMBLER]: Likewise.
	* sysdeps/mips/ieee754/bits/floatn.h [!__ASSEMBLER]: Likewise.
	* sysdeps/powerpc/bits/floatn.h [!__ASSEMBLER]: Likewise.
	* sysdeps/x86/bits/floatn.h [!__ASSEMBLER]: Likewise.