Replace remaining references to 'struct ucontext' with 'ucontext_t'

This fixes issues with glibc-2.26.

See https://bugs.gentoo.org/show_bug.cgi?id=628782 ,
https://sourceware.org/git/?p=glibc.git;h=251287734e89a52da3db682a8241eb6bccc050c9 , and
https://sourceware.org/ml/libc-alpha/2017-08/msg00010.html for context.

Change-Id: If606dc01c2172eda4ea1bc82cee6b5516cfc9253
6 files changed