add support for prefix/suffix strings to go around GlobalValue(s)

(which may or be function pointers) in the asmprinter. For the moment,
this changes nothing, except the IA64 backend which can use this to write:

  data8.ua  @fptr(blah__blah__mangled_function_name)

  (by setting FunctionAddrPrefix/Suffix to "@fptr(" / ")")

llvm-svn: 21024