[rubygems/rubygems] Fix resolution hanging on musl platforms

After recent musl support was added, Bundler started hanging in musl
platforms. I identified the issue where valid candidates were being
filtered out because their platform was specified as a string, and thus
`Gem::Platform.match_spec?` which under the hood ends up calling
`Gem::Platform#===` would return `nil`, because it does not support
comparing platforms to strings.

In particular, `Bundler::EndpointSpecification`'s platform coming from
the API was not instantiated as a `Gem::Platform`, hence the issue.

Also, this spec surfaced another issue where a bug corrected in
`Gem::Platform#match_platforms` had not been yet backported to Bundler.
So this commit also backports that to get the spec green across RubyGems
versions.

Finally, the fix in `Bundler::EndpointSpecification` made a realworld
spec start failing. This spec was faking out `rails-4.2.7.1` requirement
on Bundler in the `Gemfile.lock` file to be `>= 1.17, < 3` when the real
requirement is `>= 1.17, < 2`. Due to the bug in
`Bundler::EndpointSpecification`, the real requirement provided by the
compact index API (recorded with VCR) was being ignored, and the
`Gemfile.lock` fake requirement was being used, which made the spec
pass. This is all expected, and to fix the issue I changed the spec to
be really realworld and don't fake any Bundler requirements.

https://github.com/rubygems/rubygems/commit/faf4ef46bc

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