hardware: tweak macOS CFLAGS.

- High Sierra and Sierra support Penryn CPUs.
- `-march` implies the relevant `-msse*` flags.

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

@@ -1,7 +1,9 @@
 module Hardware
   def self.oldest_cpu
-    if MacOS.version >= :sierra
+    if MacOS.version >= :mojave
       :nehalem
+    elsif MacOS.version >= :sierra
+      :penryn
     else
       generic_oldest_cpu
     end

Library/Homebrew/hardware.rb

@@ -7,7 +7,8 @@ module Hardware
 
     class << self
       OPTIMIZATION_FLAGS = {
-        nehalem: "-march=nehalem -msse4.2",
+        nehalem: "-march=nehalem",
+        penryn:  "-march=penryn",
         core2:   "-march=core2",
         core:    "-march=prescott",
         armv6:   "-march=armv6",