Set Context::currStack when changing fibres in gdb debugging

Mohammed Bilal Akhtarrequested to merge
mbakhtar/KOS:bugfix-stackcontext into master

Currently if you run info fibres right after setidx, the output is incorrect since currStack doesn't change alongside the other registers - so we end up putting the currently-set-ed fibre's backtrace in the place where the currently running fibre's would have been.

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