[yaml2obj] - Simplify implementation. NFCI.

Knowing about how types are declared for 32/64 bit platforms:
https://github.com/llvm-mirror/llvm/blob/master/include/llvm/BinaryFormat/ELF.h#L28

it is possible to simplify code that writes a binary a bit.
The patch does that.

Differential revision: https://reviews.llvm.org/D58441

llvm-svn: 354462