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New code for SIGILL faults help identify misbranches

Contributed by Janne Johansson on 2024-02-22 from the don't pee on
the electric fence dept.

If you run recent OpenBSD on certain amd64 or aarch64 platforms,
indirect branching to an "unexpected" location will crash your
program, in order to prevent ROP attacks and similar ways to have
your program execute code where it shouldn't.

The OpenBSD compiler will insert an extra instruction in all the
places where a branch is supposed to land, and if it lands anywhere
else, a CPU fault is raised and your program gets an "Illegal
Instruction".

Previously, crashes of this kind have looked more or less like any
other kind of fault where code is executing random data or from
random locations, but since the kernel knows when this has happened,
we can make it explicit that the fault is due to missing branch
target instructions, which will help a lot when debugging.

Link to the commit here.

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