ARM: unify symbol name retrieval

Ensure that all paths that retrieve the symbol name go through GetARMGVSymbol
rather than getSymbol.  This is desirable so that any global symbol mangling can
be centralised to this function.  The motivation for this is handling of symbols
that are marked as having dll import dll storage.  Such a symbol requires an
extra load that is currently handled in the backend and a __imp_ prefix on the
symbol name.

llvm-svn: 212429