cmake: make TARGET optional in set_linker_property()

The `check_set_linker_property()` and `set_linker_property()` takes a
target argument. Make the target argument optional and use the target
`linker` as default target.

The function name `set_linker_property()` already implies that we are
setting a property and the linker target.

Remove the need to specify `TARGET linker` when using the default linker
property target.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>