a68: fix type of flex.sub_offset in struct encoded_mode

The type of flex.sub_offset in struct encoded_mode shall of course be
uint64_t rather than uint8_t.  This was triggering corrupted exports
sections once a certain amount of modes were reached.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>

gcc/algol68/ChangeLog

	* a68-imports.cc (struct encoded_mode): Type of flex.sub_offset
	shall be uint64_t.