Always support float128 on x86.

Always support float128 on x86.

In <https://gcc.gnu.org/ml/gcc-bugs/2016-08/msg03233.html>, Nick
reported i386-elf and ia64-elf failing to build because of
float128_type_node being NULL, but being used by the back end for
__float128.

The global float128_type_node is only available conditionally, if
target hooks indicate TFmode is not only available as a scalar mode
and of the right format, but also supported in libgcc.  The back-end
support, however, expects the type always to be available for
__float128 even if the libgcc support is missing.

Although a target-specific node could be restored in the case where
libgcc support is missing, it seems better to address the missing
libgcc support.  Thus, this patch enables TFmode soft-fp in libgcc
globally for all x86 targets - the only special cases needed being for
targets that use soft-fp for SFmode and DFmode, one of which already
had the support for TFmode as well (so I based the i[34567]86-*-rtems*
configuration on that present for i[34567]86-*-elfiamcu).  The i386
implementation of TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P is then
removed as no longer needed.

I can provide such a patch for ia64 if useful, but am not in a
position to test it (and while I'm reasonably confident that enabling
this support would be right for ia64-elf and ia64-freebsd, I've no
real idea if enabling libgcc support for TFmode, with or without also
enabling it for XFmode, would be safe for ia64-vms).

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc:
	* config/i386/i386.c (ix86_libgcc_floating_mode_supported_p)
	(TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P): Remove.
	* config/i386/i386elf.h (IX86_MAYBE_NO_LIBGCC_TFMODE): Likewise.
	* config/i386/lynx.h (IX86_MAYBE_NO_LIBGCC_TFMODE): Likewise.
	* config/i386/netbsd-elf.h (IX86_MAYBE_NO_LIBGCC_TFMODE):
	Likewise.
	* config/i386/netbsd64.h (IX86_MAYBE_NO_LIBGCC_TFMODE): Likewise.
	* config/i386/nto.h (IX86_MAYBE_NO_LIBGCC_TFMODE): Likewise.
	* config/i386/openbsd.h (IX86_MAYBE_NO_LIBGCC_TFMODE): Likewise.
	* config/i386/rtemself.h (IX86_NO_LIBGCC_TFMODE): Likewise.
	* config/i386/vxworks.h (IX86_MAYBE_NO_LIBGCC_TFMODE): Likewise.

libgcc:
	* config.host (i[34567]86-*-* | x86_64-*-*): Enable TFmode soft-fp
	where not already enabled.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@239775 138bc75d-0d04-0410-961f-82ee72b054a4