[yaml2obj/obj2yaml] - Allow setting an arbitrary values for e_machine.

Currently we only allow using a known named constants
for `Machine` field in YAML documents.

This patch allows using any numbers (valid or "unknown")
and adds test cases for current and new functionality.

With this it is possible to write a test cases for really unknown
EM_* targets.

Differential revision: https://reviews.llvm.org/D67652

llvm-svn: 372108