Fix: Crystal::System::Process.fork exists on windows

The method exists but raises NotImplementedError which causes
Process.new(pid) to fail because it doesn't exist with a `PidT`). This
patch fixes the issue by checking LibC.fork directly.

Weirdly, both were used before these changes and compilation did succeed.

src/compiler/crystal/compiler.cr

@@ -546,7 +546,7 @@ module Crystal
         return all_reused
       end
 
-      {% if !Crystal::System::Process.class.has_method?("fork") %}
+      {% if !LibC.has_method?("fork") %}
         raise "Cannot fork compiler. `Crystal::System::Process.fork` is not implemented on this system."
       {% elsif flag?(:preview_mt) %}
         raise "Cannot fork compiler in multithread mode"