Set up all GEMS before mruby core tasks definition

Until now, GEMs dependent on GEMs described in the build configuration file were loaded and set up after mruby core tasks were defined.
This caused an issue where, if C++ exceptions were enabled later by a dependent GEM, the necessary tasks for mruby core were not defined.

fixed https://github.com/mruby/mruby/issues/6615

lib/mruby/gem.rb

@@ -401,7 +401,16 @@ module MRuby
         end
       end
 
-      def generate_gem_table build
+      def setup(build)
+        gemset = nil
+        begin
+          gemset_prev = gemset
+          self.each(&:setup)
+          gemset = self.setup_dependencies(build).keys.sort
+        end until gemset == gemset_prev
+      end
+
+      def setup_dependencies(build)
         gem_table = each_with_object({}) { |spec, h| h[spec.name] = spec }
 
         default_gems = {}
@@ -424,6 +433,12 @@ module MRuby
           end
         end
 
+        gem_table
+      end
+
+      def generate_gem_table(build)
+        gem_table = setup_dependencies(build)
+
         each do |g|
           g.dependencies.each do |dep|
             name = dep[:gem]