Rename struct ucontext tag (bug 21457).

Rename struct ucontext tag (bug 21457).

The ucontext_t type has a tag struct ucontext.  As with previous such
issues for siginfo_t and stack_t, this tag is not permitted by POSIX
(is not in a reserved namespace), and so namespace conformance means
breaking C++ name mangling for this type.

In this case, the type does need to have some tag rather than just a
typedef name, because it includes a pointer to itself.  This patch
uses struct ucontext_t as the new tag, so the type is mangled as
ucontext_t (the POSIX *_t reservation applies in all namespaces, not
just the namespace of ordinary identifiers).  Another reserved name
such as struct __ucontext could of course be used.

Because of other namespace issues, this patch does not by itself fix
bug 21457 or allow any XFAILs to be removed.

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

	[BZ #21457]
	* sysdeps/arm/sys/ucontext.h (struct ucontext): Rename to struct
	ucontext_t.
	* sysdeps/generic/sys/ucontext.h (struct ucontext): Likewise.
	* sysdeps/i386/sys/ucontext.h (struct ucontext): Likewise.
	* sysdeps/m68k/sys/ucontext.h (struct ucontext): Likewise.
	* sysdeps/mips/sys/ucontext.h (struct ucontext): Likewise.
	* sysdeps/unix/sysv/linux/aarch64/sys/ucontext.h (struct
	ucontext): Likewise.
	* sysdeps/unix/sysv/linux/alpha/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/arm/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/hppa/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/ia64/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/m68k/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/mips/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/nios2/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/powerpc/sys/ucontext.h (struct
	ucontext): Likewise.
	* sysdeps/unix/sysv/linux/s390/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/sh/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/sparc/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/tile/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/unix/sysv/linux/x86/sys/ucontext.h (struct ucontext):
	Likewise.
	* sysdeps/powerpc/powerpc32/backtrace.c (struct
	rt_signal_frame_32): Likewise.
	* sysdeps/powerpc/powerpc64/backtrace.c (struct signal_frame_64):
	Likewise.
	* sysdeps/unix/sysv/linux/aarch64/kernel_rt_sigframe.h (struct
	kernel_rt_sigframe): Likewise.
	* sysdeps/unix/sysv/linux/aarch64/sigcontextinfo.h (SIGCONTEXT):
	Likewise.
	* sysdeps/unix/sysv/linux/arm/register-dump.h (register_dump):
	Likewise.
	* sysdeps/unix/sysv/linux/arm/sigcontextinfo.h (SIGCONTEXT):
	Likewise.
	* sysdeps/unix/sysv/linux/hppa/profil-counter.h
	(__profil_counter): Likewise.
	* sysdeps/unix/sysv/linux/microblaze/sigcontextinfo.h
	(SIGCONTEXT): Likewise.
	* sysdeps/unix/sysv/linux/mips/kernel_rt_sigframe.h (struct
	kernel_rt_sigframe): Likewise.
	* sysdeps/unix/sysv/linux/nios2/kernel_rt_sigframe.h (struct
	kernel_rt_sigframe): Likewise.
	* sysdeps/unix/sysv/linux/nios2/sigcontextinfo.h (SIGCONTEXT):
	Likewise.
	* sysdeps/unix/sysv/linux/sh/makecontext.S (__makecontext):
	Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/makecontext.c
	(__start_context): Likewise.
	* sysdeps/unix/sysv/linux/tile/sigcontextinfo.h (SIGCONTEXT):
	Likewise.
	* sysdeps/unix/sysv/linux/x86_64/register-dump.h (register_dump):
	Likewise.
	* sysdeps/unix/sysv/linux/x86_64/sigcontextinfo.h (SIGCONTEXT):
	Likewise.