[yaml2obj] - Add a support for changing EntSize.

I was trying to add a test case for LLD and found that it
is impossible to set sh_entsize via yaml.
The patch implements the missing part.

Differential revision: https://reviews.llvm.org/D50235

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339113 91177308-0d34-0410-b5e6-96231b3b80d8