superenv: use Hardware::CPU.universal_archs

This defines the new HOMEBREW_ARCHS environment variable, which is
currently only set during universal builds, so that the tool wrappers
no longer need to hardcode i386/x86_64.

Library/ENV/4.3/cc

@@ -165,7 +165,7 @@ class Cmd
       args << '-march=native' if tool =~ /clang/
     end
 
-    args += %w{-arch i386 -arch x86_64} if cccfg? 'u'
+    args += ENV['HOMEBREW_ARCHS'].split(',').map {|a| "-arch #{a}"} if cccfg? 'u'
     args << "-std=#{@arg0}" if @arg0 =~ /c[89]9/
     args
   end

Library/Homebrew/superenv.rb

@@ -86,6 +86,7 @@ class << ENV
   end
 
   def universal_binary
+    ENV['HOMEBREW_ARCHS'] = Hardware::CPU.universal_archs.join(',')
     append 'HOMEBREW_CCCFG', "u", ''
   end