[HWASan] Catch cases where libc populated jmp_buf.

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
to it.

See discussion on https://reviews.llvm.org/D109699 and
https://reviews.llvm.org/D69045.

Fixes https://github.com/google/sanitizers/issues/1244.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D109787