Closes Homebrew/homebrew#21733.
Closes Homebrew/homebrew#21782.

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

Fixes Homebrew/homebrew#21943.

This defines the new HOMEBREW_ARCHS environment variable, which is
currently only set during universal builds, so that the tool wrappers
no longer need to hardcode i386/x86_64.

Also adds some reusable constants into the global Hardware::CPU
namespace, available on both OS X and Linux.

Replaced the plethora of ternaries we've used all over the place to
determine whether x86_64 or i386 is called for.

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

Up to now a brewed Python, even if not linked, was preferred over
and external Python, so that you had to completely `brew rm python`
to have a formula link against external (mostly system) Python.

From now on it is okay to `brew unlink python`.

Now, we can finally stop stuggesting to
    `brew rm <foo>` and then `brew install <foo> <with-your-preferred options>

So `brew reinstall` will honor all options that have been recorded
into the INSTALL_RECEIPT.json plus if `--build-bottle` was used.

If the `src` in the Cellar of a formula is a symlink to another dir
in the same Cellar (for example `Frameworks/QtGui.framework` is a
symlink to `lib/QtGui.framework`), then we should make a symlink
directly instead of possibly creating an empty dir.

This is needed because Ruby's `find` does not follow symlinks (newer
versions can, but we have to support 10.8 for a long time).

Currently this probably only occurs for Qt, where we put symlinks
into the Cellar/Frameworks and the frameworks linking code is a bit
special to create a directory to "merge" together different versions
of the same framework from different formulae. See `python` and
`python3`; both install `Python.framework` and only one can be the
`Current` but the other should still be put into
`Python.framework/Versions`. Therefore `Python.framework` has to
be a dir and no symlink.

Return value was used only by one test, rewrote test to know expected value.