gmap-gsnap: Add variant for different cpu support. (#8087)

* gmap-gsnap: Add variant for different cpu support.

* gmap-gsnap: fix flake8 error.

* gmap-gsnap: remove sys import.

* Change variant type to multi-valued.

* remove if-statement.

* gmap-gsnap: Build in seperate directories.

* gmap-gsnap: remove make distclean.

* gmap-gsnap: rewrite the recipe.