feat: add process termination codes and SYS_ABORT; whole-process abort() from any thread in libc and montauk-rt

programs/lib/libc/libc.c

@@ -130,6 +130,7 @@ static inline long _mtk_syscall4(long nr, long a1, long a2, long a3, long a4) {
 #define SYS_GETENVIRON 171
 #define SYS_SETENVIRON 172
 #define SYS_SPAWN_ENV 173
+#define SYS_ABORT     198
 
 /* ========================================================================
    errno
@@ -1243,9 +1244,9 @@ void exit(int status) {
     __builtin_unreachable();
 }
 
+/* Ends the whole process from any thread; waitpid sees WTERMSIG == SIGABRT. */
 void abort(void) {
-    _mtk_syscall1(SYS_PRINT, (long)"abort() called\n");
-    _mtk_syscall1(SYS_EXIT, 1);
+    _mtk_syscall0(SYS_ABORT);
     __builtin_unreachable();
 }
 
@@ -4055,8 +4056,8 @@ pid_t waitpid(pid_t pid, int *status, int options) {
     }
     long code = _mtk_syscall1(SYS_WAITPID, (long)pid);
     if (status != NULL) {
-        if (code >= 256) {
-            *status = (int)((code - 256) & 0x7F);   /* signaled */
+        if (code >= TerminationBase) {
+            *status = (int)((code - TerminationBase) & 0x7F);   /* signaled */
         } else {
             *status = (int)((code & 0xFF) << 8);    /* exited */
         }