[PowerPC] ELFv2 dynamic loader support

This patch enables the new ELFv2 ABI in the runtime dynamic loader.
The loader has to implement the following features:
- In the ELFv2 ABI, do not look up a function descriptor in .opd, but
  instead use the local entry point when resolving a direct call.
- Update the TOC restore code to use the new TOC slot linkage area
  offset.
- Create PLT stubs appropriate for the ELFv2 ABI.

Note that this patch also adds common-code changes. These are necessary
because the loader must check the newly added ELF flags: the e_flags
header bits encoding the ABI version, and the st_other symbol table
entry bits encoding the local entry point offset.  There is currently
no way to access these, so I've added ObjectFile::getPlatformFlags and
SymbolRef::getOther accessors.

Reviewed by Hal Finkel.