Remove LLVM_GLOBAL_VISIBILITY, which is unused, and was not working properly.

The problem was that the test for whether a compiler supports it or not was
inaccurate, but has to be accurate: LLVM_LOCAL_VISIBILITY is an optimization
and not needed for correctness, so wrongly thinking a compiler doesn't support
it is not a big deal, but LLVM_GLOBAL_VISIBILITY is for correctness, and not
an optimization: getting it wrong is fatal: it needs to be set based on a
configure test not testing the gcc version.  Since dragonegg has moved to a
different scheme, and it was the only user of LLVM_GLOBAL_VISIBILITY, just
remove this macro.

llvm-svn: 113959