Set target-cpu through RUSTFLAGS

Library/Homebrew/extend/ENV/shared.rb

@@ -28,6 +28,7 @@ module SharedEnvExtension
     CMAKE_PREFIX_PATH CMAKE_INCLUDE_PATH CMAKE_FRAMEWORK_PATH
     GOBIN GOPATH GOROOT PERL_MB_OPT PERL_MM_OPT
     LIBRARY_PATH LD_LIBRARY_PATH LD_PRELOAD LD_RUN_PATH
+    RUSTFLAGS
   ].freeze
   private_constant :SANITIZED_VARS
 

Library/Homebrew/extend/ENV/std.rb

@@ -36,6 +36,7 @@ module Stdenv
     self["PKG_CONFIG_LIBDIR"] = determine_pkg_config_libdir
 
     self["MAKEFLAGS"] = "-j#{make_jobs}"
+    self["RUSTFLAGS"] = Hardware.rustflags_target_cpu
 
     if HOMEBREW_PREFIX.to_s != "/usr/local"
       # /usr/local is already an -isystem and -L directory so we skip it