security: ARM does not scrub registers when returning from system calls

[andrewboie]

_arm_do_syscall(), upon returning from executing the system call handler, does not erase the contents of general purpose registers. This can leak sensitive supervisor-only data to user mode.

[andrewboie]

As far as I can tell, we need to zero out r1, r2, and r3.
r4 and above will be automatically be restored to original state by the syscall handler per the C calling convention

[andrewboie]

I added a fix for this to #7752