[dev.link] cmd/link: on AIX, use relocation to locate target symbol from TOC symbol
On AIX, a TOC symbol always has a relocation to its target symbol.
Instead of using name lookup to locate the target symbol, we can
just use the relocation.
Using name lookup, besides being less efficient, needs to provide
the right symbol version. In this particular case, we are looking
for a data symbol so it is almost always version 0. But in case
that it is a text symbol, we may get an ABIALIAS symbol, which
does not have its Sect set.