cpu: Fix a bug in getCurrentInstCount in the checker CPU.

cpu: Fix a bug in getCurrentInstCount in the checker CPU.

An earlier change accidentally left out the actualTC-> prefix in the
getCurrentInstCount method which was supposed to delegate the call to
another thread context. Without that, it just called itself and would
infinitely recurse.

This bug was pointed out in email by Robert Henry.

Change-Id: Ibf1fee6b48ff87790309c6d435bd76fa95c6cab9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22623
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>