Some setjmp calls within libc cannot be intercepted while their matching
longjmp calls can be. This causes problems if our setjmp/longjmp
interceptors don't use the exact same format as libc for populating and
reading the jmp_buf.
We add a magic field to our jmp_buf and populate it in setjmp. This
allows our longjmp interceptor to notice when a libc jmp_buf is passed
See discussion on https://reviews.llvm.org/D109699 and