Add a MCAsmInfoELF class and factor some code into it.

We had a MCAsmInfoCOFF, but no common class for all the ELF MCAsmInfos before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192760 91177308-0d34-0410-b5e6-96231b3b80d8