use packaged gem files mode as hint only

proposal to fix one of the problems in #1040, this allows systems `umask` to be applied properly to extracted gem files

to fully avoid #1002 it would be required to know `spec.executables` so the `775` could be forced on those files
this is archived with https://github.com/rubygems/rubygems/pull/1041/files#diff-3a0baa1500616100ad7469f0f7b20c3cR403