Add a workaround for missing call to cross_compile block

[larskanis]

The cross_compiling block wasn't called for the x86_64-linux platform, leading to an additional dependency to mini_portile2 and missing post install message.

Related patch to rake-compiler is here: rake-compiler/rake-compiler#171

Fixes #1991

[flavorjones]

@larskanis Do you think the change in rake-compiler/rake-compiler#171 will make it into a rake-compiler release sometime soon?

This change, while effective, feels dirty and so I think I'd prefer to bump the dependency in Nokogiri (or even just build locally using rake-compiler master) ... but I don't feel strongly about that.

[larskanis]

@flavorjones IMHO this patch isn't different to other patches we apply to upstream libraries. I'll take care that this issue will be fixed in the next rake-compiler version, but I wouldn't want to block nokogiri until then.

The grpc.gem have had a whole bunch of such patches to the rake-compiler-dock container. In the meantime I solved most of them in the upstream projects (rake-compiler, rake-compiler-dock and ruby), so that only one is left.