[MsgPack] Add support for writing binary ELF doc nodes

Will be used for a front-end compiler
https://github.com/GPUOpen-Drivers/llpc
to insert a binary blob value into AMDGPU PAL metadata.

Differential Revision: https://reviews.llvm.org/D148607

Change-Id: I90685883d1478335937218e6b1a569b1b26dc05a