[yaml2obj][obj2yaml] Support custom ELF section header string table name

This patch adds support for a new field in the FileHeader, which states
the name to use for the section header string table. This also allows
combining the string table with another string table in the object, e.g.
the symbol name string table. The field is optional. By default,
.shstrtab will continue to be used.

This partially fixes https://bugs.llvm.org/show_bug.cgi?id=50506.

Reviewed by: Higuoxing

Differential Revision: https://reviews.llvm.org/D104035