Add missing nothrow @nogc to signal functions and callWithStackShell callback

This is a backport of the druntime commits
cb78abd053615f8f59da7d85ccb0f5486381aa32: add missing nothrow @nogc to signal functions
ffc39c96b476bd1b7d2202bca870a4f06c241b32: require nothrow for callWithStackShell callback

They are required to compile druntime on ARM platform.