cpu/cortexm_common: make SVC call always available

[dylad]

Contribution description

This PR proposes to make SVC call always available for user and thus removes the cortexm_svc pseudomodule. SVCall is available on all ARMvx-M architecture. This exception was used before to trigger PendSV for the sheduler. Now PendSV is triggered manually and no longer require to use SVCall. Since the feature was provided by default for all cortexm, I think it makes sense to remove the pseudomodule and to let it hardcoded in the code.

Testing procedure

CI should be happy.

Issues/PRs references

None.