Support: split object format out of environment

This is a preliminary setup change to support a renaming of Windows target
triples.  Split the object file format information out of the environment into a
separate entity.  Unfortunately, file format was previously treated as an
environment with an unknown OS.  This is most obvious in the ARM subtarget where
the handling for macho on an arbitrary platform switches to AAPCS rather than
APCS (as per Apple's needs).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203160 91177308-0d34-0410-b5e6-96231b3b80d8