Use -march=native for non-bottle source builds [Linux]

Library/Homebrew/extend/os/extend/ENV/shared.rb

@@ -1 +1,5 @@
-require "extend/os/mac/extend/ENV/shared" if OS.mac?
+if OS.mac?
+  require "extend/os/mac/extend/ENV/shared"
+elsif OS.linux?
+  require "extend/os/linux/extend/ENV/shared"
+end

Library/Homebrew/extend/os/linux/extend/ENV/shared.rb

@@ -0,0 +1,10 @@
+module SharedEnvExtension
+  # @private
+  def effective_arch
+    if ARGV.build_bottle?
+      ARGV.bottle_arch || Hardware.oldest_cpu
+    else
+      :native
+    end
+  end
+end

Library/Homebrew/hardware.rb

@@ -7,6 +7,7 @@ module Hardware
 
     class << self
       OPTIMIZATION_FLAGS = {
+        native:  "-march=native",
         nehalem: "-march=nehalem",
         penryn:  "-march=penryn",
         core2:   "-march=core2",