a68: fetch module exports from packet by name

a68: fetch module exports from packet by name

A packet (compilation unit) may emit more than one module interface in
its exports section.  This is because a module may publicize the
exports of other module.  This commit makes the import infrastructure
to read multiple module interfaces from exports sections and then look
for the accessed module in the data.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>
gcc/algol68/ChangeLog

	* a68-types.h (struct MOIF_T): Add chain_next to GTY info.
	* a68-imports.cc (a68_decode_modes): Mode offsets are relative to
	the start of the moif, not the start of the exports.
	(a68_decode_moifs): Renamed from a68_decode_moif and changed to
	decode multiple moifs from the exports.
	(a68_open_packet): Call a68_decode_moifs and look for the right
	moif.
	* a68-exports.cc (a68_moif_new): Initialize NEXT (moif).

gcc/testsuite/ChangeLog

	* algol68/execute/modules/module17.a68: New test.