Fix sparc64 bits/setjmp.h namespace (bug 21261).

sysdeps/unix/sysv/linux/sparc/bits/setjmp.h defines 64-bit __jmp_buf
with a load of identifiers that are not part of any standard
namespace, resulting in conform/ tests failing.  This patch fixes this
by moving those identifiers to the implementation namespace, so
enabling the conform/ tests to pass for sparc64.

Tested (compilation only) for sparc64 with build-many-glibcs.py.

	[BZ #21261]
	* sysdeps/unix/sysv/linux/sparc/bits/setjmp.h
	[__WORDSIZE == 64 && !_ASM] (__sparc64_jmp_buf): Use reserved
	names for all fields.
	* sysdeps/sparc/sparc64/jmpbuf-unwind.h (_JMPBUF_UNWINDS): Update
	for jmp_buf field renaming.
	(_JMPBUF_UNWINDS_ADJ): Likewise.