Add float128 support for ia64.

Add float128 support for ia64.

This patch enables float128 support for ia64, so that all the
configurations where GCC supports _Float128 / __float128 as an
ABI-distinct type now have glibc support as well.  bits/floatn.h
declares the support to be available for GCC 4.4 and later, which is
when the libgcc support was added.  The removal of
sysdeps/ia64/fpu/k_rem_pio2.c is because the generic k_rem_pio2.c
defines a function required by the float128 code.

Tested (compilation only) with build-many-glibcs.py for ia64 (GCC 6
and GCC 7).

Given how long it is since libm-test-ulps has been updated for ia64, I
think truncating the file and regenerating it from scratch would be a
good idea when doing a regeneration to add float128 ulps.  I expect
various ia64 libm issues (at least some already filed in Bugzilla) to
result in test failures even after ulps regeneration, but hopefully
the float128 code will pass tests as it's the same as used on other
architectures.

	* sysdeps/ia64/Implies: Add ieee754/float128.
	* sysdeps/ia64/bits/floatn.h: New file.
	* sysdeps/ia64/float128-abi.h: Likewise.
	* manual/math.texi (Mathematics): Document support for _Float128
	on ia64.
	* sysdeps/ia64/Makefile [$(subdir) = math] (CPPFLAGS): Append to
	Makefile variable.
	* sysdeps/ia64/fpu/e_sqrtf128.c: New file.
	* sysdeps/ia64/fpu/k_rem_pio2.c: Remove file.
	* sysdeps/ia64/fpu/sfp-machine.h: New file.  Based on libgcc.
	* sysdeps/ia64/math-tests.h: New file.
	* math/libm-test-support.h (XFAIL_FLOAT128_PAYLOAD): Also define
	based on TEST_COND_binary128 for [__ia64__].
	* sysdeps/unix/sysv/linux/ia64/libc.abilist: Update.
	* sysdeps/unix/sysv/linux/ia64/libm.abilist: Likewise,