devel/llvm-{cheri,cheriot,morello}: assorted fixups

Allow override of GH_PROJECT and GH_ACCOUNT[0] and fix GH_PROJECT in
llvm-cheriot. No change in the source, but the filename is now
corrected.

Centralize include of Makefile.snapshot.

Reported by: Irina Dudina <irina.dudina@ed.ac.uk>