RFC-9: Add new capability for seL4 SMC Forwarding

I’d like to solicit discussion on the new RFC on forwarding Secure Monitor Calls on Arm:
https://sel4.atlassian.net/browse/RFC-9

Relevant Arm documentation (SMC calling conventions):
https://developer.arm.com/documentation/den0028/latest?_ga=2.116565828.390371079.1616755184-1989679030.1616755184

Any opinions, concerns, alternative designs?