Closes Homebrew/homebrew#42354.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

`MacOS::Xcode.without_clt? && MacOS.sdk_path.nil?` should never be true.

In its earliest form, this would raise a bare RuntimeError in an effort
to have the bug reported. Later, it was changed to silently disable
superenv. But we don't want to do that. If there's a bug, or the user's
system is misconfigured, we want to know, so that we can fix the bug, or
the user can fix their system. So let's remove the condition.

This replaces hardcoding of i386/x86_64 all over the code.

Now that this is handled by a Requirement with a default formula, this
code cannot be triggered under normal circumstances.

The advice given has now been moved to the Formula Cookbook on the wiki.

This allows formulae which won't build with Tiger's ld to conditionally
request a dependency on the ld64 formula. This modifies the build
environment appropriately, and will only be active on Tiger.

The depends_on 'nose' => :python

tests fails even if people did `pip install nose`
with their brewed pythons because during
the tests the PATH is set up such that
/usr/bin comes before HOMEBRE_PREFIX/bin.

This is a hot fix. Proper fix need resorting
the PATH.

Fixes Homebrew/homebrew#19013.
Fixes Homebrew/homebrew#19862.
Fixes Homebrew/homebrew#19921.

The CFLAGS were previously not generic enough.

References Homebrew/homebrew#18944.
References Homebrew/homebrew#19179.

* CPU functions now exist in Hardware::CPU
* Added compatibility functions in compat/hardware_compat.rb
* Names are less specific to Mac hardware, e.g. CPU.family instead of
  Hardware.intel_family
* Hardware::CPU.family works for both Intel and PowerPC
* New helper methods on CPU, like .sse4? and .altivec?

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>