[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372108 91177308-0d34-0410-b5e6-96231b3b80d8