ruby: Fall back to RbConfig::CONFIG's arch if rubyarchhdrdir is not set.

Follow-up to 6a128750: rubyarchhdrdir is only defined from Ruby 2.0 on,
so the header detection would not work correctly if one is using an
earlier version. Instead, first try rubyarchhdrdir and, if the directory
does not exist, fall back to the previous behavior of using arch.
Merge pull request #8 from rakuco/ruby19-arch-detection

ruby: Fall back to RbConfig::CONFIG's arch if rubyarchhdrdir is not set.