Move three methods only used by MCJIT to MCJIT.

These methods are only used by MCJIT and are very specific to it. In fact, they
are also fairly specific to the fact that we have a dynamic linker of
relocatable objects.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223964 91177308-0d34-0410-b5e6-96231b3b80d8