[PPC64] Add lazy symbol resolution stubs.

Adds support for .glink resolver stubs from the example implementation in the V2
ABI (Section 4.2.5.3. Procedure Linkage Table). The stubs are written to the
PltSection, and the sections are renamed to match the PPC64 ABI:
    .got.plt --> .plt    Type = SHT_NOBITS
    .plt     --> .glink

And adds the DT_PPC64_GLINK dynamic tag to the dynamic section when the plt is
not empty.

Differential Revision: https://reviews.llvm.org/D45642

llvm-svn: 331840