[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