make: Fix slink_reg.sv target

[colluca]

If $(SLINK_ROOT)/src/regs/slink_reg.sv is deleted but $(SLINK_ROOT)/src/regs/slink_reg_pkg.sv is not, the prior won't be rebuilt.

This is exactly what happens in Gwaihir. The target is more correctly written as suggested, albeit as a consequence it runs twice.

The only solution to both problems would be to use "Grouped Targets", but these have only been introduced since Make 4.3.

[fischeti]

Thanks! LGTM now. The CI is failing because of some spurious issues with the environment, but this should be fixed if you rebase onto main.