Fix uc_* namespace (bug 21457).

Fix uc_* namespace (bug 21457).

The standard members of ucontext_t, in all standard versions with that
type, are uc_link, uc_sigmask, uc_stack and uc_mcontext.

The uc_* namespace is mostly reserved for additions to the structure.
However, in XPG4.2, it's only reserved when <ucontext.h> is included,
not when <signal.h> is included, while <signal.h> is required to
define ucontext_t (but not allowed to make visible other symbols from
<ucontext.h>).  Thus, nonstandard members should avoid uc_* names.
Some already do use __uc_*, but others don't; most architectures (all
except ia64, I think) have a member uc_flags and some have additional
members beyond that.

This patch makes nonstandard members have an __ prefix unless
__USE_MISC is defined.  Members whose names indicate they are solely
padding / reserved for future use are renamed unconditionally to use
the __glibc_reserved1 naming convention.

This is part of the preparation for a revised version of the
mcontext_t / sigcontext patch to be able to eliminate all 13 of the
miscellaneous XFAILs in conform/Makefile, rather than only 11 of them
as at present (at least one further fix on top of this one will be
needed for that as well).

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

	[BZ #21457]
	* sysdeps/arm/sys/ucontext.h (__ctx): Move undefine further down.
	(ucontext_t): Use __ctx with uc_flags.  Rename uc_filler to
	__glibc_reserved1.
	* sysdeps/generic/sys/ucontext.h (__ctx): New macro.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/i386/sys/ucontext.h (__ctx): Move undefine further down.
	(__ctxt): Likewise.
	(ucontext_t): Use __ctx with uc_flags.  Rename uc_filler to
	__glibc_reserved1.
	* sysdeps/m68k/sys/ucontext.h (__ctx): Move undefine further down.
	(ucontext_t): Use __ctx with uc_flags.  Rename uc_filler to
	__glibc_reserved1.
	* sysdeps/mips/sys/ucontext.h (__ctx): Move undefine further down.
	(ucontext_t): Use __ctx with uc_flags.  Rename uc_filler to
	__glibc_reserved1.
	* sysdeps/unix/sysv/linux/aarch64/sys/ucontext.h (__ctx): New
	macro.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/unix/sysv/linux/alpha/sys/ucontext.h (__ctx): New macro.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/unix/sysv/linux/arm/sys/ucontext.h (__ctx): New macro.
	(ucontext_t): Use __ctx with uc_flags and uc_regspace.
	* sysdeps/unix/sysv/linux/hppa/sys/ucontext.h (__ctx): New macro.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/unix/sysv/linux/m68k/sys/ucontext.h (__ctx): Move
	undefine further down.
	(ucontext_t): Use __ctx with uc_flags.  Rename uc_filler to
	__glibc_reserved1.
	* sysdeps/unix/sysv/linux/mips/sys/ucontext.h (__ctx): Move
	undefine further down.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/unix/sysv/linux/nios2/sys/ucontext.h (__ctx): Move
	undefine further down.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/unix/sysv/linux/powerpc/sys/ucontext.h (ucontext_t): Use
	__ctx with uc_flags, uc_regs_ptr, uc_regs and uc_reg_space.
	Rename uc_pad to __glibc_reserved1.
	* sysdeps/unix/sysv/linux/s390/sys/ucontext.h (__ctx): Move
	undefine further down.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/unix/sysv/linux/sh/sys/ucontext.h (__ctx): Move undefine
	further down.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/unix/sysv/linux/sparc/sys/ucontext.h (ucontext_t): Use
	__ctx with uc_flags.
	* sysdeps/unix/sysv/linux/tile/sys/ucontext.h (__ctx): New macro.
	(ucontext_t): Use __ctx with uc_flags.
	* sysdeps/unix/sysv/linux/x86/sys/ucontext.h (ucontext_t): Use
	__ctx with uc_flags.