Improved archive (of bitcode modules) linking performance for

LLVM >= 3.3 by effectively reimplementing the linking algorithm
used in LLVM <= 3.2.

The LLVM specific bitcode archive format has been removed
from LLVM >= 3.3 . Now archives are normal system archives that can
contain LLVM bitcode modules as well as regular binary object files.

The previous commit implemented an approach where ALL the bitcode
modules get linked in which can be terribly slow when klee-uclibc gets
linked (~600 LLVM modules).

Here are the options that I considered to address this:

* Use LD with LLVM gold plug-in and call as an external program.
  I Don't really want to add another dependency to KLEE. It already
  has enough!

* Use the upcomming LLVM linker (lld). Not really an option
  because at the time of writing there is no support for linking
  archives of bitcode modules.

* Don't use archives at all and just work with modules (i.e.
  replace uses of llvm-ar with llvm-link and tinker with the
  flags a little). This isn't so great because the resulting
  LLVM bitcode module we execute is bigger than it should be.

* Reimpelent bitcode archive linking ourselves in a slightly
  better way.

I've gone for the last option

This implementation unfortunately loads all bitcode modules into memory
first so we can query the module symbols tables. I would prefer to read
the archive's index and link in modules on demand but unfortunately
although the new Object::Archive interface in LLVM allows iteration over
symbols it doesn't provide a way of knowing if that symbol is
defined/undefined.

This implementation is far from perfect!

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