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

This relands
https://chromium.googlesource.com/breakpad/breakpad/src/+/e3035bc406cee8a4d765e59ad46eb828705f17f4,
which was accidentally committed to breakpad/breakpad/src, the read-only
mirror of src in breakpad/breakpad. (Well, it should have been
read-only.) See https://crbug.com/766164.

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: Id66f474d636dd2afa450bab925c5514a800fdd6f
Reviewed-on: https://chromium-review.googlesource.com/674304
Reviewed-by: Mark Mentovai <mark@chromium.org>

Cr-Mirrored-From: https://chromium.googlesource.com/breakpad/breakpad
Cr-Mirrored-Commit: bddcc58860f522a0d4cbaa7e9d04058caee0db9d
6 files changed