Sound_QSchedule doesn't enforce X bit when used in SWI form

This isn’t a bug as such, but it does cause the system to crash rather spectacularly:

Repro scenario:

1. Schedule SWI’s via Sound_QSchedule to a Music Module, but don’t set the X bit
2. RMKill the Music Module
3. When the Scheduler calls the next scheduled SWI, it generates an “SWI not known” error, corrupts the stack and crashes the system

Alternative repro:

1. Schedule an SWI via Sound_QSchedule using an unknown SWI
2. When the Scheduler calls the next scheduled SWI, it generates an “SWI not known” error, corrupts the stack and crashes the system

In the interests of OS stability, its probably worth forcing the X bit on scheduled SWI’s to ensure that an unknown SWI, or SWI that returns with V set, doesn’t crash the system.