[ELF] Explicitly write null bytes in string tables

Following r309829, if a string table appears in an executable segment, the strings
will not be null terminated. This is a problem, for example, for the .dynstr
section when using -no-rosegment. The strings end up being terminated with 0xcc
because prior to this patch, LLD did not explicitly write the null terminators.
This change fixes that by always writing the null terminators.

Reviewers: rafael

Differential Revision: https://reviews.llvm.org/D36267

llvm-svn: 310042