[Beignet] wrong CMAKE_LIBRARY_ARCHITECTURE on x86_64

[Rebecca N. Palmer]

That the file gets named intel-beignet-.icd rather than 
intel-beignet.icd is a purely cosmetic issue: it should work either way, 
but won't support multiarch, as the names of .icd files need to be 
distinct for co-installability but are otherwise irrelevant.

If you want working multiarch, you'll need to actually set 
CMAKE_LIBRARY_ARCHITECTURE (or with the patch, 
BEIGNET_LIBRARY_ARCHITECTURE), e.g. with cmake's -D option; again it 
doesn't matter what you set it to, as long as the values are distinct.