bc files for modules with a target triple that indicates they are for
darwin. The reader unconditionally handles this, and the writer could
turn this on for more targets if we care.
This change has two benefits for darwin:
1) it allows us to encode the cpu type of the file in an easy to read
place that doesn't require decoding the bc file.
2) it works around a bug (IMO) in darwin's AR where it is incapable of
handling files that are not a multiple of 8 bytes long. BC files
are only guaranteed to be multiples of 4 bytes long.
llvm-svn: 53275
