More OpenBSD fixes

Summary:
- Use internal_syscall_ptr in internal_readlink
- use sigcontext on OpenBSD

Patch by David CARLIER

Reviewers: krytarowski, vitalybuka

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D44713

llvm-svn: 328239