[obj2yaml] - SHT_DYNAMIC and SHT_REL* sections: stop dumping sh_entsize field...

[obj2yaml] - SHT_DYNAMIC and SHT_REL* sections: stop dumping sh_entsize field when it has the default value.

Currently obj2yaml always emits the `EntSize` property when `sh_entsize != 0`.
It is not correct. For example, for `SHT_DYNAMIC` section, `EntSize == 0`
is abnormal, while `sizeof(ELFT::Dyn)` is the expected default.

To reduce the output produces we should not dump default values.

yaml2obj tests that shows `sh_entsize` values produced are:
1) For `SHT_REL*` sections: `yaml2obj\ELF\reloc-sec-entry-size.yaml`
2) For `SHT_DYNAMIC`: `yaml2obj\ELF\dynamic-section.yaml`

Differential revision: https://reviews.llvm.org/D76227