Add a copy of missing <ucontext.h> for Android and enable ASan SEGV handler.

This change adds a copy of <ucontext.h> for Android found in google-breakpad
that is missing from the official NDK.
ASan SEGV handler is still disabled by default and can be enabled with
ASAN_OPTIONS=handle_segv.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201084 91177308-0d34-0410-b5e6-96231b3b80d8