MC: restore behaviour of defaulting to ELF

This restores the previous behaviour of just assuming that if you dont specify a
valid triple that you really meant the default triple with an ELF object file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207349 91177308-0d34-0410-b5e6-96231b3b80d8