update GLIBC error message

[flavorjones]

Currently the minimum glibc is different between x86* and arm64 linux:

#rakelib/extensions.rake
  def dll_ref_versions
    case platform
    when X86_LINUX_PLATFORM_REGEX
      { "GLIBC" => "2.17" }
    when ARM_LINUX_PLATFORM_REGEX
      { "GLIBC" => "2.29" }
    else
      raise "CrossRuby.dll_ref_versions: unmatched platform: #{platform}"
    end
  end

However, the error message emitted by lib/nokogiri/extension.rb specifies just the x86* requirement. That error message should be made more clear.

Originally reported by @joeyparis at #2359 (comment)

[flavorjones]

PR #2428 and sparklemotion/nokogiri.org@9481914 update the error message and documentation.