[yaml2elf] - Support describing .stack_sizes sections using unique suffixes.

Currently we can't use unique suffixes in section names to describe
stack sizes sections. E.g. '.stack_sizes [1]' will be treated as a regular section.
This happens because we recognize stack sizes section by name and
do not yet drop the suffix before the check.

The patch fixes it.

Differential revision: https://reviews.llvm.org/D68018

llvm-svn: 372853