[Sanitizer] Fix build openbsd

- The alternative syscall exists only on FreeBSD.
- Adding OpenBSD's equivalent and while at it other
remaining oses ones.

Reviewers: vitalybuka, krytarowsky, hiraditya

Reviewed By: hiraditya

Differential Revision: https://reviews.llvm.org/D50760

llvm-svn: 339886