lld: set sh_link and sh_info for .rela.plt sections

ELF spec says that for SHT_REL and SHT_RELA sh_link should reference the
associated string table and sh_info should reference the "section to
which the relocation applies." ELF Tool Chain's elfcopy / strip use
this (in part) to control whether or not the relocation entry is copied
to the output.

LLVM PR 37538 https://bugs.llvm.org/show_bug.cgi?id=37538

Approved by: re (kib)
Obtained from: llvm r344226 (backported for 6.0)