compat: correct MacOS.prefer_64_bit? deprecation

[jonchang]

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Reported via the Discourse forum. It seems like #5477 had a small bug where MacOS.prefer_64_bit? was prematurely disabled by being inaccessible.

Steps to reproduce:

echo 'MacOS.prefer_64_bit?' | brew irb

Incorrect behavior:

$ echo 'MacOS.prefer_64_bit?' | brew irb
==> Interactive Homebrew Shell
Example commands available with: brew irb --examples
Switch to inspect mode.
MacOS.prefer_64_bit?
NoMethodError: undefined method `prefer_64_bit?' for OS::Mac:Module
	from (irb):1
	from /usr/local/Homebrew/Library/Homebrew/brew.rb:88:in `<main>'

Correct behavior:

$ echo 'MacOS.prefer_64_bit?' | brew irb
==> Interactive Homebrew Shell
Example commands available with: brew irb --examples
Switch to inspect mode.
MacOS.prefer_64_bit?
MethodDeprecatedError: Calling MacOS.prefer_64_bit? is deprecated! There is no replacement.
	from /usr/local/Homebrew/Library/Homebrew/compat/os/mac.rb:6:in `prefer_64_bit?'
	from (irb):1
	from /usr/local/Homebrew/Library/Homebrew/brew.rb:88:in `<main>'

[jonchang]

Going to merge this since this is an active bug and tests have passed.

[MikeMcQuaid]

Thanks for the fix @jonchang!