kernel/svc: Reorganize svcSetThreadCoreMask()

Makes the code much nicer to follow in terms of behavior and control
flow. It also fixes a few bugs in the implementation.

Notably, the thread's owner process shouldn't be accessed in order to
retrieve the core mask or ideal core. This should be done through the
current running process. The only reason this bug wasn't encountered yet
is because we currently only support running one process, and thus every
owner process will be the current process.

We also weren't checking against the process' CPU core mask to see if an
allowed core is specified or not.

With this out of the way, it'll be less noisy to implement proper
handling of the affinity flags internally within the kernel thread
instances.