Terminal: Redraw entire line if any of its characters are dirty.

This means we only have to do one fill_rect() per line and the whole process
ends up being ~10% faster than before.

Also added a read_tsc() syscall to give userspace access to the TSC.

LibC/unistd.cpp

@@ -306,4 +306,10 @@ void sync()
     syscall(SC_sync);
 }
 
+int read_tsc(unsigned* lsw, unsigned* msw)
+{
+    int rc = syscall(SC_read_tsc, lsw, msw);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
+
 }