Common shouldn't be depending on APIs in Core (in this, case depending
on the PowerPC namespace). Because of the poor separation here, this
moves OSThread functionality into core, so that it resolves the implicit
dependency on core.
Given this is what occurs in both constructors (as one just passes
through to another), we can just initialize the member directly.
While we're at it, amend the struct to follow the general ordering
convention of:
<new types>
<functions>
<variables>
