arch,sim: Merge initCPU and startupCPU.

arch,sim: Merge initCPU and startupCPU.

These two functions were called in exactly one place one right after
the other, and served similar purposes.

This change merges them together, and cleans them up slightly. It also
removes checks for FullSystem, since those functions are only called
in full system to begin with.

Change-Id: I214f7d2d3f88960dccb5895c1241f61cd78716a8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24904
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>