[ELF] - Do not use HeaderSize for conditions in PltSection.

Previously we checked (HeaderSize == 0) to find out if
PltSection section is IPLT or PLT. Some targets does not set
HeaderSize though. For example PPC64 has no lazy binding implemented
and does not set PltHeaderSize constant.

Because of that using of both IPLT and PLT relocations worked
incorrectly there (testcase is provided).

Patch fixes the issue.

Differential revision: https://reviews.llvm.org/D41613

llvm-svn: 322362