[stackprotector] Add in the stackprotector libcall.

We support this libcall on all platforms except for OpenBSD (See
lib/Codegen/StackProtector.cpp).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188193 91177308-0d34-0410-b5e6-96231b3b80d8