[ELF2/PPC64] Resolve local-call relocations using the correct function-descriptor values

Under PPC64 ELF v1 ABI, the symbols associated with each function name don't
point directly to the code in the .text section (or similar), but rather to a
function descriptor structure in a special data section named .opd. The
elements in the .opd structure include a pointer to the actual code, and a the
relevant TOC base value. Both of these are themselves set by relocations.

When we have a local call, we need the relevant relocation to refer directly to
the target code, not to the function-descriptor in the .opd section. Only when
we have a .plt stub do we care about the address of the .opd function
descriptor itself.

So we make a few changes here:

 1. Always write .opd first, so that its relocated data values are available
    for later use when writing the text sections. Record a pointer to the .opd
    structure, and its corresponding buffer.

 2. When processing a relative branch relocation under ppc64, if the
    destination points into the .opd section, read the code pointer out of the
    function descriptor structure and use that instead.

This this, I can link, and run, a dynamically-compiled "hello world"
application on big-Endian PPC64/Linux (ELF v1 ABI) using lld.

llvm-svn: 250122