Syscall and call should be allowed in kernel modules

[plafer]

While looking into #1680, I noticed that the assembler rejects syscalls and calls in kernel modules. If I am not mistaken, this is too strict: we should allow calls/syscalls in the root context if we're not processing a syscall (i.e. in_syscall flag is false). It is also inconsitent that dyncall is currently allowed, but not call and syscall.

Hence, I think we should remove that check from the assembler.

[bobbinth]

I think allowing call should be pretty straight-forward (since dyncall is allowed already), but syscall may require more thinking. The main concern is about how we handle fmp register when performing a syscall. It actually may be fine as is, but we need to make sure it works as expected.