[zircon][profiles] Wire priority set

.. for the Profile object. There are no real callers yet.

A smoke test has been added. There is no current way to
introspect on the thread priority so more work to come.

Change-Id: I577b52d59ac4a004681b7743d8d33e9d7039ae2c

[zircon] syscall boilerplate for profile

Introduces the two new syscalls
1- creates a profile object
2- set a profile to an existng object (must be a thread atm)

There is no actual meat.

Change-Id: Iebdacc04a6291520678661bda785858ca3815070