Fix mcontext_t sigcontext namespace (bug 21457)

[Joseph Myers]

On Tue, 27 Jun 2017, Carlos O'Donell wrote:

> On hppa, ia64, arm, alpha, aarch64, and tile, they do have a
> `typedef struct sigcontext mcontext_t;` which would make this work.

And microblaze (using sysdeps/generic/sys/ucontext.h before this patch), 
for which my architecture maintainer ping bounced.

-- 
Joseph S. Myers
joseph@codesourcery.com