[ORC] Add a method to JITTargetMachineBuilder to get the default data layout

[ORC] Add a method to JITTargetMachineBuilder to get the default data layout
for the target machine.

This simplifies usage during setup of concurrent JIT stacks where the client
needs a DataLayout, but not a TargetMachine (TargetMachines are created on
the fly by the compile threads later).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343429 91177308-0d34-0410-b5e6-96231b3b80d8