emulators/flycast: populate GH_TUPLE manually

Only three git submodules are currently relevant. This can be handled
manually to simplify the port:

• remove Makefile.tuples file and target
• remove ${FILESDIR}/gen_gh_tuple.sh
• add GH_TUPLE to Makefile

PORTREVISION bump not required because this doesn't change how the
port is built.

PR: 288008
Reported by: Stefan Schlosser <bsdcode@disroot.org> (maintainer)