[ELF] - Do not produce broken .dynsym with --no-rosegment.

We produce broken output currently.
Code that writes .dynsym assumes output buffer is zero-filled,
though that is not always true. When --no-rosegment is given,
buffer can be filled with trap instructions. Patch fixes the issue.

It is relative with PR34705.

Differential revision: https://reviews.llvm.org/D38579

llvm-svn: 315053