Always support float128 on ia64 (PR target/77586).

Always support float128 on ia64 (PR target/77586).

Bug 77586, and previously
<https://gcc.gnu.org/ml/gcc-bugs/2016-08/msg03233.html>, reports
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 ia64 targets.  Support for XFmode in libgcc (that is,
for libgcc2.c XFmode functions, not soft-fp) is also enabled for all
ia64 targets so that ia64 no longer needs to define the
TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P hook.

I've confirmed that ia64-elf builds cc1 with this patch and it passes
-fself-test.  I have not otherwise tested the patch.  It's plausible
that ia64-elf and ia64-freebsd might work as-is, but ia64-vms probably
needs further changes, by someone familiar with VMS shared libraries,
to implement an equivalent of ia64/t-softfp-compat in that case
(avoiding conflicts between __divtf3 from soft-fp and the old alias
for __divxf3).

	PR target/77586
gcc:
	* config/ia64/ia64.c (ia64_libgcc_floating_mode_supported_p)
	(TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P): Remove.
	* config/ia64/elf.h (IA64_NO_LIBGCC_TFMODE): Likewise.
	* config/ia64/freebsd.h (IA64_NO_LIBGCC_TFMODE): Likewise.
	* config/ia64/vms.h (IA64_NO_LIBGCC_XFMODE)
	(IA64_NO_LIBGCC_TFMODE): Likewise.

libgcc:
	* config.host (ia64*-*-elf*, ia64*-*-freebsd*, ia64-hp-*vms*): Use
	soft-fp.

From-SVN: r240955