Add getter method to access Reloc::Model.

Some MC components like Target Streamers or Assembly Parsers
may need to access the relocation model in order to expand
some directives and/or assembly macros.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202418 91177308-0d34-0410-b5e6-96231b3b80d8