[sanitizer] Use ucontext_t instead of "struct ucontext".

Fixes Mac build.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@178755 91177308-0d34-0410-b5e6-96231b3b80d8
[sanitizer] Android lacks ucontext_t definition.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@178756 91177308-0d34-0410-b5e6-96231b3b80d8