tools/memory-model: Make checkghlitmus.sh use mselect7

The checkghlitmus.sh script currently uses grep to ignore non-C-language
litmus tests, which is a bit fragile.  This commit therefore enlists the
aid of "mselect7 -arch C", given Luc Maraget's recent modifications that
allow mselect7 to operate in filter mode.

This change requires herdtools 7.52-32-g1da3e0e50977 or later.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>