Support syscall6
Redox currently supports syscall0..=syscall5, i.e. rax+rdi+rsi+rdx+r10+r8, but some future syscalls like preadv2/pwritev2 (and futex?) on 32-bit architectures would need e.g. SYS_PWRITEV2+fd+addr+len+off_lo+off_hi+flags, i.e. 7 args.
The registers Linux uses for that, are
- x86_64: rax, rdi, rsi, rdx, r10, r8, r9
- x86_32: eax, ebx, ecx, ecx, edi, esi, ebp
- aarch64: x8, x0, x1, x2, x3, x4, x5, x6
Might be worth looking into whether supporting full-width syscall return values, on x86/x86_64, by setting the carry flag, improves performance (the BSDs do this IIRC).