hardware: use Nehalem flags on >= Sierra.

See https://en.wikipedia.org/wiki/MacOS_Sierra#System_requirements.

Library/Homebrew/extend/os/hardware.rb

@@ -1,4 +1,5 @@
 if OS.mac?
+  require "extend/os/mac/hardware"
   require "extend/os/mac/hardware/cpu"
 elsif OS.linux?
   require "extend/os/linux/hardware/cpu"

Library/Homebrew/extend/os/mac/hardware.rb

@@ -0,0 +1,9 @@
+module Hardware
+  def self.oldest_cpu
+    if MacOS.version >= :sierra
+      :nehalem
+    else
+      generic_oldest_cpu
+    end
+  end
+end

Library/Homebrew/hardware.rb

@@ -7,6 +7,7 @@ module Hardware
 
     class << self
       OPTIMIZATION_FLAGS = {
+        nehalem: "-march=nehalem -msse4.2",
         core2:   "-march=core2",
         core:    "-march=prescott",
         armv6:   "-march=armv6",
@@ -130,7 +131,8 @@ module Hardware
     end
   end
 
-  def self.cores_as_words
+  class << self
+    def cores_as_words
       case Hardware::CPU.cores
       when 1 then "single"
       when 2 then "dual"
@@ -143,7 +145,7 @@ module Hardware
       end
     end
 
-  def self.oldest_cpu
+    def oldest_cpu
       if Hardware::CPU.intel?
         if Hardware::CPU.is_64_bit?
           :core2
@@ -160,6 +162,8 @@ module Hardware
         Hardware::CPU.family
       end
     end
+    alias generic_oldest_cpu oldest_cpu
+  end
 end
 
 require "extend/os/hardware"