libc: io: temporarily switch stderr to line-buffered mode

no-buffering isn't compatible with kern_log (used by bootstrap), as it
automatically appends a newline to every string written. this will
be switched back once kern_log isn't being used anymore.

lib/libc/io/stdio/stderr.c

@@ -5,7 +5,9 @@
 static struct __opaque_file __stderr = {
 	.f_fd = 2,
 	.f_flags = FILE_STATIC,
-	.f_buffer_mode = _IONBF,
+	/* TODO change this back to _IONBF once we're no longer using kern_log()
+         * in bootstrap */
+	.f_buffer_mode = _IOLBF,
 };
 
 extern struct __opaque_file *__libc_file_stderr(void)