[MC] Don't use the architecture to govern which object file format to use

[MC] Don't use the architecture to govern which object file format to use

InitMCObjectFileInfo was trying to override the triple in awkward ways.
For example, a triple specifying COFF but not Windows was forced as ELF.
This makes it easy for internal invariants to get violated, such as
those which triggered PR25912.

This fixes PR25912.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256226 91177308-0d34-0410-b5e6-96231b3b80d8