[obj2yaml] - Dump the `EShNum` key in some cases.

This patch starts emitting the `EShNum` key, when the `e_shnum = 0`
and the section header table exists.

`e_shnum` might be 0, when the the number of entries in the section header
table is larger than or equal to SHN_LORESERVE (0xff00).
In this case the real number of entries
in the section header table is held in the `sh_size`
member of the initial entry in section header table.

Currently, obj2yaml crashes when an object has `e_shoff != 0` and the `sh_size`
member of the initial entry in section header table is `0`.
This patch fixes it.

Differential revision: https://reviews.llvm.org/D92098