[RuntimeDyld, PowerPC] Fix check for external symbols when detecting reloction overflow

The PowerPC part of processRelocationRef currently assumes that external
symbols can be identified by checking for SymType == SymbolRef::ST_Unknown.
This is actually incorrect in some cases, causing relocation overflows to
be mis-detected. The correct check is to test whether Value.SymbolName
is null.

Includes test case. Note that it is a bit tricky to replicate the exact
condition that triggers the bug in a test case. The one included here
seems to fail reliably (before the fix) across different operating
system versions on Power, but it still makes a few assumptions (called
out in the test case comments).

Also add ppc64le platform name to the supported list in the lit.local.cfg
files for the MCJIT and OrcMCJIT directories, since those tests were
currently not run at all.

Fixes PR32650.

Reviewer: hfinkel

Differential Revision: https://reviews.llvm.org/D33402

llvm-svn: 303637