Refactor ildasm round trip logic

Refactor ildasm round trip logic

This change factors the Ildasm round trip logic so that it should not
require a separate test build. The ildasmroundtrip flag is no longer
passed to build-test.cmd; instead the wrapper scripts are always
generated with a check for an environment variable that can be set from
runtests.

Also remove separate test build for ilrt in the ci scripts

Commit migrated from https://github.com/dotnet/coreclr/commit/f60551763ccabdeca6171b55957e5a059f038818