std: use HOMEBREW_ARCH env var on Linux

Unlike macOS, Linux installations can be installed/deployed on a
variety of hardware. The HOMEBREW_ARCH environment variable
allows the end user to set a custom -march=... option for the
compiler. If the variable is not set, it defaults to "native".

This only applies on Linux; on macOS, behavior remains unchanged.

Library/Homebrew/extend/ENV/std.rb

@@ -7,7 +7,8 @@ module Stdenv
 
   # @private
   SAFE_CFLAGS_FLAGS = "-w -pipe".freeze
-  DEFAULT_FLAGS = "-march=core2 -msse4".freeze
+  HOMEBREW_ARCH = (ENV["HOMEBREW_ARCH"] || "native").freeze
+  DEFAULT_FLAGS = (OS.mac? ? "-march=core2 -msse4" : "-march=#{HOMEBREW_ARCH}").freeze
 
   # @private
   def setup_build_environment(formula = nil)