Fix tags targets in module/Makefile.in + cleanup

These targets look to have been copied from an automake-generated
Makefile.in, and can't work since none of the auto-generated automake
variables are defined here.

Moreover, ctags has been overridden in the top-level Makefile, so the
target is pointless anyway, and gtags is not a recursive target.

Fix cscopelist by moving it to the top-level Makefile as well, in line
with ctags and etags.

Also, add -a to ctags command as well, otherwise it won't work if more
than one xargs invocation takes place.

Add assembler files to ctags/etags, prune all dotted-dirs, and restrict
the find to files only.

Cleanup: add .PHONY to module/Makefile.in, and fix one recipe with a
missing continuation character.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10493