Story 4.5.2 — Syscall stub implementation

[Abhinkop]

Goal

Implement the newlib syscall stubs so that malloc, printf, and free work.

Tasks

• _sbrk: heap break pointer backed by kmalloc pages
• _write: route fd=1/2 to UART via sys_write
• _exit: call sys_exit
• _getpid: call sys_getpid
• _read, _open, _close, _lseek, _fstat, _isatty: return -ENOSYS stubs
• _kill: return -ENOSYS stub