envutil.rb: backtrace may not be present

* test/ruby/envutil.rb (assert_separately): SystemStackError at
  machine stack overflow on platforms where sigaltstack is
  unavailable does not have backtrace.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44130 b2dd03c8-39d4-4d8f-98ff-823fe69b080e