a68: handle Darwin's #1/NN archive members

In Darwin archives use an extension to encode the member names
off-header, in the first bytes of the contents.  The name in the
header proper is then #1/NN... where NN... is an ascii-encoded number
in decimal with the length of the name, which is encoded right after
the header as part of the contents.

This patch fixes the Algol 68 module importing from archives in Darwin
by acknowledging this extension.  Note that, for the moment, we don't
bother to fetch the actual member name.  This can lead to poor
diagnostics, so we probably will fix that at some point.

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

gcc/algol68/ChangeLog

	* a68-imports-archive.cc (Archive_file): Handle Darwin #1/NN
	archive members.