Kernel: Stack pointer is validated when updated

This allows us not to fail stack pointer when in syscall since
interrupts use their own stack

kernel/include/kernel/Syscall.h

@@ -10,9 +10,9 @@ namespace Kernel
 {
 
 	template<typename T1 = void*, typename T2 = void*, typename T3 = void*>
-	inline int syscall(int syscall, T1 arg1 = nullptr, T2 arg2 = nullptr, T3 arg3 = nullptr)
+	inline long syscall(int syscall, T1 arg1 = nullptr, T2 arg2 = nullptr, T3 arg3 = nullptr)
 	{
-		int ret;
+		long ret;
 		asm volatile("int $0x80" : "=a"(ret) : "a"(syscall), "b"((uintptr_t)arg1), "c"((uintptr_t)arg2), "d"((uintptr_t)arg3) : "memory");
 		return ret;
 	}