fix: Gem::Platform.match handles String argument properly

Previously 9eead86 introduced non-commutativity of platforms, and
later commit 1b9f7f5 changed the behavior of `Gem::Platform.match` to
ensure the callee of `#=~` was the gem platform.

However, when the platform argument is a String, then the callee and
argument of `#=~` are flipped (see docs for `String#=~`), which works
against the fix from 1b9f7f5.

Closes rubygems#5938

lib/rubygems/platform.rb

@@ -22,6 +22,7 @@ def self.match(platform)
   end
 
   def self.match_platforms?(platform, platforms)
+    platform = Gem::Platform.new(platform) unless platform.is_a?(Gem::Platform)
     platforms.any? do |local_platform|
       platform.nil? ||
         local_platform == platform ||

test/rubygems/test_gem_platform.rb

@@ -452,6 +452,13 @@ def test_inspect
     assert_equal 1, result.scan(/@version=/).size
   end
 
+  def test_gem_platform_match_with_string_argument
+    util_set_arch "x86_64-linux-musl"
+
+    assert(Gem::Platform.match(Gem::Platform.new("x86_64-linux")), "should match Gem::Platform")
+    assert(Gem::Platform.match("x86_64-linux"), "should match String platform")
+  end
+
   def assert_local_match(name)
     assert_match Gem::Platform.local, name
   end