RUBY_PLATFORM contains the build-time kernel version, not runtime
RUBY_PLATFORM contains the build-time kernel version, not runtime

* https://github.com/ruby/spec/runs/365115886
  1)
  RUBY_PLATFORM contains the current kernel major version FAILED
  Expected "x86_64-darwin18" =~ /19/

ruby/spec,nobu/rubyspec,eregon/rubyspec,ruby/spec,ruby/spec,eregon/rubyspec,eregon/rubyspec,nobu/rubyspec,nobu/rubyspec