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The supported version of the Android NDK now provides sys/ucontext.h for
all architectures, and we no longer need to provide our own definitions.

Keep the ones used by PNaCl but update the defines to no longer use them
on Android.

Bug: 437330
Change-Id: Ia46c746b17b72261842284d4fed3772fea3a96ef
Reviewed-on: https://chromium-review.googlesource.com/1252243Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595181}