This test was already very close to being an ordinary codegen test, except that
it needed some extra logic to set a few variables based on (target) platform
characteristics.
Now that we have support for `//@ filecheck-flags:`, we can instead set those
variables using the normal test revisions mechanism.
This makes room for migrating over `tests/run-make/instrument-coverage`,
without increasing the number of top-level items in the codegen test directory.