Add support for -march=sandybridge

HHVM's been building with this (or equivalent predecessor patches); it
gets us a 20x speedup on real-world workloads, at the cost of slightly
worse compatibility.

This doesn't change the default, just makes it available as an explicit
choice.

Library/Homebrew/hardware.rb

@@ -14,6 +14,7 @@ module Hardware
         @optimization_flags ||= {
           native:             arch_flag("native"),
           nehalem:            "-march=nehalem",
+          sandybridge:        "-march=sandybridge",
           core2:              "-march=core2",
           core:               "-march=prescott",
           arm_vortex_tempest: "",