Improve syscall definitions

- Allow constant id specification
- Define function signature in SYSCALL macro
- Move implementation into src/kernel/syscalls/*.cpp

src/kernel/syscall.h

@@ -6,12 +6,20 @@ struct cpu_state;
 
 enum class syscall : uint64_t
 {
-#define SYSCALL(name, nargs) name ,
+#define SYSCALL(id, name, result, ...) name = id,
 #include "syscalls.inc"
 #undef SYSCALL
 
-	COUNT
+	// Maximum syscall id. If you change this, also change
+	// MAX_SYSCALLS in syscall.s
+	MAX = 64
 };
 
 void syscall_enable();
-extern "C" void syscall_invalid(uint64_t call);
+
+namespace syscalls
+{
+#define SYSCALL(id, name, result, ...) result name (__VA_ARGS__);
+#include "syscalls.inc"
+#undef SYSCALL
+}