sched: Implement basic scheduling and signal handling system

Note: this is probably 25% broken, but it works right now as written, so I hope it all works.

- Added a new scheduler header file (scheduler.h) defining task structures, scheduling policies, and signal handling mechanisms.
- Integrated scheduling functions into the syscall interface, including SYS_GETPID, SYS_GETPPID, SYS_EXIT, SYS_SCHED_YIELD, SYS_NICE, SYS_KILL, SYS_SIGACTION, SYS_SIGPROCMASK, SYS_SCHED_GETSCHEDULER, and SYS_SCHED_SETSCHEDULER.
- Updated syscall handler to manage new scheduling-related syscalls and signal actions.

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/syscalls.h

@@ -4,16 +4,26 @@
 #define SYS_WRITE  1
 #define SYS_OPEN   2
 #define SYS_CLOSE  3
+#define SYS_SCHED_YIELD    24
 
-static inline long syscall(long num, long a1, long a2, long a3)
+static inline long syscall(long num,
+                            unsigned long a1,
+                            unsigned long a2,
+                            unsigned long a3,
+                            unsigned long a4,
+                            unsigned long a5,
+                            unsigned long a6)
 {
     long ret;
-
     asm volatile (
+        "mov %4, %%r10\n"
+        "mov %5, %%r8\n"
+        "mov %6, %%r9\n"
         "syscall"
         : "=a"(ret)
-        : "a"(num), "D"(a1), "S"(a2), "d"(a3)
+        : "a"(num), "D"(a1), "S"(a2), "d"(a3),
+          "r"(a4), "r"(a5), "r"(a6)
+        : "r10", "r8", "r9", "rcx", "r11", "memory"
     );
-
     return ret;
 }