common: Don't try to use an undefined SIGSTKFLT
common: Don't try to use an undefined SIGSTKFLT

Just check if the signal name  is defined, like for `SIGEMT` and others. This fixes build failures on alpha and sparc where that signal does not exist.

Closes #8728
cockpituous/cockpit,garrett/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,andreasn/cockpit,mvollmer/cockpit,cockpit-project/cockpit,stefwalter/cockpit,deryni/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,andreasn/cockpit,andreasn/cockpit,martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,mareklibra/cockpit,mareklibra/cockpit,andreasn/cockpit,mareklibra/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,deryni/cockpit,cockpituous/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpituous/cockpit,stefwalter/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,cockpit-project/cockpit,deryni/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,andreasn/cockpit,deryni/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,stefwalter/cockpit,cockpit-project/cockpit,mareklibra/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,andreasn/cockpit,mareklibra/cockpit,cockpituous/cockpit,andreasn/cockpit,stefwalter/cockpit,mareklibra/cockpit