Improve syscall definitions

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

src/kernel/syscalls.inc

@@ -1,12 +1,12 @@
-SYSCALL(noop,     0)
-SYSCALL(exit,     1)
-SYSCALL(getpid,   0)
-SYSCALL(fork,     0)
+SYSCALL(0x00, noop,     void)
+SYSCALL(0x01, exit,     void, int64_t)
+SYSCALL(0x02, getpid,   pid_t)
+SYSCALL(0x03, fork,     pid_t)
 
-SYSCALL(message,  1)
+SYSCALL(0x10, message,  void, const char *)
 
-SYSCALL(pause,    0)
-SYSCALL(sleep,    1)
+SYSCALL(0x20, pause,    void)
+SYSCALL(0x21, sleep,    void, uint64_t)
 
-SYSCALL(send,     2)
-SYSCALL(receive,  2)
+SYSCALL(0x30, send,     void)
+SYSCALL(0x31, receive,  void)