Generalize linkDynLib for ELF platforms

To create a dynamic shared object (DSO) on ELF platforms, we invoke
"gcc -shared". This in turn invokes ld. We supply -Bsymbolic to the
linker, as "ld -Bsymbolic -shared -o <lib> <objs>" resolves all
references from <objs> to <objs> at library creation time.

See http://hackage.haskell.org/trac/ghc/wiki/Commentary/PositionIndependentCode