fix: CTRL+C/CTRL+Z job control and doom build errors

fix: CTRL+C/CTRL+Z job control and doom build errors

1. CTRL+C/CTRL+Z: Shell now calls setsid() instead of setpgid(0,0)
   to create a proper session. This initializes tty_session_id so
   TIOCSPGRP can actually set child processes as the foreground
   group. Previously, TIOCSPGRP silently returned -EPERM because
   tty_session_id was 0.

2. Doom mkdir: Added mkdir/stat/fstat/chmod declarations to
   user/ulibc/include/sys/stat.h where POSIX expects them.
   Doom's m_misc.c includes sys/stat.h for mkdir().

3. Doom __divdi3: Added libgcc.a to doom link step to provide
   compiler runtime helpers for 64-bit arithmetic on i386.