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

[David Miller]

From: Joseph Myers <joseph@codesourcery.com>
Date: Fri, 17 Mar 2017 03:08:58 +0000

> 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.

This is going to break the build of the address sanitizer on sparc64.

It will also probably break lots of other programs too that grovel
through the mcontext register bits for unwinding and state inspection.