Fix unwind registration for multiple JIT instances in a single process.

[lhames]

/cherry-pick 84fe1f6

This is the fix for #126611. It only affects ORC, and only changes behavior when used with libunwind (so primarily on Darwin and BSD, but some Linux clients too).

[llvmbot]

/pull-request #126825

[lhames]

/cherry-pick 84fe1f6 9f61a60

Add fix for unused variable warning.

[llvmbot]

@llvm/issue-subscribers-orcjit

Author: Lang Hames (lhames)

/cherry-pick 84fe1f6

This is the fix for #126611. It only affects ORC, and only changes behavior when used with libunwind (so primarily on Darwin and BSD, but some Linux clients too).