microkernel: Fibers and ISRs may invoke microkernel no-op

Adds support that allows fibers and ISRs to invoke the microkernel
no-op kernel service request. This is useful for cases when the
nanokernel needs to invoke the microkernel task scheduler.

Change-Id: I1f4b2a39ac6b5e44bb1b6c6b3cd6034262bbada8
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>