[RuntimeDyld, PowerPC] Fix regression from r303637

Actually, to identify external symbols, we need to check for
*either* non-null Value.SymbolName *or* a SymType of
Symbol::ST_Unknown.

The former may happen for symbols not known to the JIT at all
(e.g. defined in a native library), while the latter happens
for symbols known to the JIT, but defined in a different module.

Fixed several regressions on big-endian ppc64.

llvm-svn: 303655