[yaml2obj] - Improve handling of SectionHeaderTable::NoHeaders flag.

When `NoHeaders` is set, we still have following issues:
1) We emit the `.shstrtab` implicit section of size 1 (empty string table).
2) We still align the start of the section header table, what affects the output size.
3) We still write section header table bytes.

This patch fixes all of these issues.

Differential revision: https://reviews.llvm.org/D90295