Allow building bottles with custom architectures

This allows overriding the "oldest CPU" behaviour - for instance, to
build Intel bottles for a newer CPU than Core 2, to build a PPC bottle
with or without Altivec on the same computer, etc.

Library/ENV/4.3/cc

@@ -157,7 +157,10 @@ class Cmd
     args = %w{-pipe -w -Os}
 
     # When bottling use the oldest supported CPU type.
-    if cccfg? 'bi6'
+    if cccfg? 'bc'
+      # Custom bottle specified during the build
+      args << ENV['HOMEBREW_ARCHFLAGS']
+    elsif cccfg? 'bi6'
       args << '-march=core2'
     elsif cccfg? 'bi'
       args << '-march=prescott'

Library/Homebrew/extend/ARGV.rb

@@ -133,6 +133,11 @@ module HomebrewArgvExtension
     include? '--build-bottle' or !ENV['HOMEBREW_BUILD_BOTTLE'].nil?
   end
 
+  def bottle_arch
+    arch = value 'bottle-arch'
+    arch.to_sym if arch
+  end
+
   def build_from_source?
     include? '--build-from-source' or !ENV['HOMEBREW_BUILD_FROM_SOURCE'].nil? \
       or build_head? or build_devel? or build_universal? or build_bottle?

Library/Homebrew/extend/ENV/std.rb

@@ -324,7 +324,8 @@ module Stdenv
     append flags, xarch unless xarch.empty?
 
     if ARGV.build_bottle?
-      append flags, Hardware::CPU.optimization_flags[Hardware.oldest_cpu]
+      arch = ARGV.bottle_arch || Hardware.oldest_cpu
+      append flags, Hardware::CPU.optimization_flags[arch]
     else
       # Don't set -msse3 and older flags because -march does that for us
       append flags, map.fetch(Hardware::CPU.family, default)

Library/Homebrew/extend/ENV/super.rb

@@ -76,6 +76,12 @@ module Superenv
     self['CMAKE_LIBRARY_PATH'] = determine_cmake_library_path
     self['ACLOCAL_PATH'] = determine_aclocal_path
 
+    # For custom bottles, need to specify the arch in the environment
+    # so that the compiler shims have access
+    if (arch = ARGV.bottle_arch)
+      self['HOMEBREW_ARCHFLAGS'] = Hardware::CPU.optimization_flags[arch]
+    end
+
     # The HOMEBREW_CCCFG ENV variable is used by the ENV/cc tool to control
     # compiler flag stripping. It consists of a string of characters which act
     # as flags. Some of these flags are mutually exclusive.
@@ -83,6 +89,7 @@ module Superenv
     # u - A universal build was requested
     # 3 - A 32-bit build was requested
     # b - Installing from a bottle
+    # c - Installing from a bottle with a custom architecture
     # i - Installing from a bottle on Intel
     # 6 - Installing from a bottle on 64-bit Intel
     # p - Installing from a bottle on PPC
@@ -237,7 +244,9 @@ module Superenv
   def determine_cccfg
     s = ""
     if ARGV.build_bottle?
-      s << if Hardware::CPU.type == :intel
+      s << if ARGV.bottle_arch
+        'bc'
+      elsif Hardware::CPU.type == :intel
         if Hardware::CPU.is_64_bit?
           'bi6'
         else