ibcore: Mark write-only variables

[andrew]

Summary

Some LinuxKPI lock macros pass need a flags field passed in. This is
written to but never read from so gcc complains.

Fix this by marking the flags variables as unused to quieten the
compiler.

Sponsored by: Arm Ltd

[brooks]

I wonder if we want a local #define __writeonly __unused for documentation purposes.

[imp]

In D45303#1037067, @brooks wrote:

I wonder if we want a local #define __writeonly __unused for documentation purposes.

If we do, maybe that's best in sys/cdefs.h instead of here?

[kib]

May be instead, the *lock_irq* macros might do something to fake the argument use?

[andrew]

Rebase on D45561