[kernel] Allow for more than three syscall args

The rcx register is used by the function call ABI for the 4th argument,
but is also clobbered by SYSCALL to hold the IP. The r10 register is
caller-saved but not part of the ABI, so stash rcx there when crossing
the syscall boundary.

src/kernel/syscall.s

@@ -23,9 +23,11 @@ syscall_handler_prelude:
 	mov rbp, rsp
 
 	; account for the hole in the sysv abi
-	; argument list since SYSCALL uses rcx
+	; argument list since SYSCALL uses rcx.
-	mov rcx, r8
+	; r10 is non-preserved but not part of
-	mov r8, r9
+	; the function call ABI, so the rcx arg
+	; was stashed there.
+	mov rcx, r10
 
 	push rbx
 	push r11

src/libraries/libc/arch/x86_64/syscalls.s

@@ -5,9 +5,10 @@
 		mov rbp, rsp
 
 		; args should already be in rdi, etc, but rcx will
-		; get stomped, so shift args out one spot from rcx
+		; get stomped, so stash it in r10, which isn't a
-		mov r9, r8
+		; callee-saved register, but also isn't used in the
-		mov r8, rcx
+		; function call ABI.
+		mov r10, rcx
 
 		mov rax, %2
 		syscall