[obj2yaml,yaml2obj] - Fix issues with creating/dumping group sections.

We have the following issues related to group sections:
1) yaml2obj is unable to set the custom `sh_entsize` value, because the `EntSize`
   key is currently ignored.
2) obj2yaml is unable to dump the group section which `sh_entsize != 4`.
3) obj2yaml always dumps the "EntSize" for group sections, though
   usually we are trying to omit dumping default values when dumping keys.
   I.e. we should not print the "EntSize" key when `sh_entsize` == 4.

This patch fixes (1),(3) and adds the test case to document the behavior of (2).

Differential revision: https://reviews.llvm.org/D93854