I've just committed a fix to src/sys/lib/libsa/dosfs.c; here's the diff,
so I don't have to type out the comment again:

+#ifdef BOOTXX
+       /* due to __internal_memset_ causing all sorts of register spillage
+          (and being completely unoptimized for zeroing small amounts of
+          memory), if we hand-initialize the remaining members of f to zero,
+          the code size drops 68 bytes. This makes no sense, admittedly. */
+       f->offset = 0;
+       f->c = 0;
+#else
        bzero(f, sizeof(DOS_FILE));
+#endif