Fix mcontext_t sigcontext namespace (bug 21457)

[Joseph Myers]

On Mon, 26 Jun 2017, Zack Weinberg wrote:

> Independent of that, this is finicky enough and has enough
> kernel-interface implications that I think it had better have
> architecture maintainer reviews for, at least, all architectures where
> you copied the contents of struct sigcontext into the definition of
> mcontext_t.

Ping of <https://sourceware.org/ml/libc-alpha/2017-06/msg00946.html> for 
architecture maintainers for aarch64, alpha, hppa, ia64, microblaze, tile.

-- 
Joseph S. Myers
joseph@codesourcery.com