tools/memory-model: Keep assembly-language litmus tests

This commit retains the assembly-language litmus tests generated from
the C-language litmus tests, appending the hardware tag to the original
C-language litmus test's filename.  Thus, S+poonceonces.litmus.AArch64
contains the Armv8 assembly language corresponding to the C-language
S+poonceonces.litmus test.

This commit also updates the .gitignore to avoid committing these
automatically generated assembly-language litmus tests.

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