signals: more general changes

kernel-src/arch/x86-64/context.asm

@@ -1,6 +1,7 @@
 section .text
 global arch_context_switch
 arch_context_switch:
+	xor rsi, rsi ; is syscall? (no)
 	extern sched_userspacecheck
 	call sched_userspacecheck ; compiled with no_caller_saved_registers
 

kernel-src/arch/x86-64/syscall.asm

@@ -226,7 +226,11 @@ arch_syscall_entry:
 
 	mov rdi, rsp
 	add rdi, 8 ; context pointer is after r9 argument
-	extern sched_userspacecheck
+	xor rsi, rsi
+	inc rsi ; is syscall? (yes)
+	; errno already in rdx
+	mov rcx, rax ; return value
+	extern sched_userspacecheck ; context, return, errno
 	call sched_userspacecheck ; compiled with __attribute__((no_caller_saved_registers))
 
 	; restore context