runtime: add debugger hook for fatal signals [freeze exception]

[hyangah]

@aarzilli 's https://go.dev/cl/495736 refactors sighandler and moves fatal signal handling code to a separate fatalsignal function.
This allows debuggers like Delve to continue to check and intercept fatal signals when necessary. It would be nice if this change is merged to go1.21.

cc @golang/release

[cagedmantis]

Approved after discussion within the group. This would help avoid a bug.

[dmitshur]

CL 495736 is submitted, closing. Please comment if something more is needed for this freeze exception.