Delete directory using symlink when decompressing tar

In 2.7.6, the safety of symlink is confirmed with `mkdir_p_safe`, Before that `FileUtils.rm_rf destination` is running. Therefore, if `tmp/dir` is specified after `tmp -> /tmp`, the following `/tmp/dir` is deleted.

require 'rubygems/package'

class GemBuiler

   def initialize spec, path
    @_build_time      = Time.now
    @_checksums       = {}
    @_signer          = Gem::Security::Signer.new nil, nil, ""
    @_spec            = spec
    @_path            = path
  end

  def build &block
    Gem.load_yaml …