Do not use gems precompiled for MRI on TruffleRuby

* See rubygems/rubygems#2945
* The platforms are set before they are read so this is safe.
  truffleruby.rb is required directly from rubygems.rb.

(cherry picked from commit 6da9d7c)

Author: eregon

CHANGELOG.md

@@ -78,6 +78,7 @@ Bug fixes:
 * Fixed linking of always-inline C API functions with `-std=gnu90` (#1837, #1879).
 * Avoid race conditions during `gem install` by using a single download thread.
 * `RSTRING_PTR()` now always returns a native pointer, resolving two bugs `memcpy`ing to (#1822) and from (#1772) Ruby Strings.
+* Do not use gems precompiled for MRI on TruffleRuby (#1837).
 
 Compatibility:
 

lib/truffle/rubygems/defaults/truffleruby.rb

@@ -16,4 +16,9 @@ module Gem
   def self.default_dir
     @default_dir ||= "#{Truffle::Boot.ruby_home or raise 'TruffleRuby home not found'}/lib/gems"
   end
+
+  # Only report the RUBY platform as supported to make sure gems precompiled for MRI are not used.
+  # TruffleRuby has a different ABI and cannot reuse gems precompiled for MRI.
+  # See https://github.com/rubygems/rubygems/issues/2945
+  Gem.platforms = [Gem::Platform::RUBY]
 end

spec/truffle/rubygems/platforms_spec.rb

@@ -0,0 +1,17 @@
+# Copyright (c) 2020 Oracle and/or its affiliates. All rights reserved. This
+# code is released under a tri EPL/GPL/LGPL license. You can use it,
+# redistribute it and/or modify it under the terms of the:
+#
+# Eclipse Public License version 2.0, or
+# GNU General Public License version 2, or
+# GNU Lesser General Public License version 2.1.
+# OTHER DEALINGS IN THE SOFTWARE.
+
+require_relative '../../ruby/spec_helper'
+require 'rubygems'
+
+describe "Gem.platforms" do
+  it "returns only [RUBY] on TruffleRuby to not use gems precompiled for MRI" do
+    Gem.platforms.should == [Gem::Platform::RUBY]
+  end
+end