[obj2yaml,yaml2obj] - Refine how we set/dump the sh_entsize field.

This reuses the code from yaml2obj (moves it to ELFYAML.h).
With it we can set the sh_entsize in a single place in obj2yaml.

Note that it also fixes a bug of yaml2obj: we do not
set the sh_entsize field for the SHT_ARM_EXIDX section properly.

Differential revision: https://reviews.llvm.org/D93858