Move the extern symbol declarations outside of

DynamicLibrary::SearchForAddressOfSymbol and force them to have "C"
linkage. 

Interestingly, GCC treats the block-scoped "extern" declarations we
previously had as if they were extern "C" declarations (or, at least,
were in the global namespace), so that GCC bug papered over this LLVM
bug. Clang and EDG get the linkage correct; this new variant seems to
work for both GCC and Clang.

llvm-svn: 92020