Xcode: enable support for 32/64-bit universal builds

SDL 1.2.14 is coming out very soon, and there's a fully functional preview release up on libsdl.org. Might as well enable 64-bit now.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

Created by  Steven Noonan  on October 11, 2009
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