Avoid caching the relocation model on the subtarget, this is for

Avoid caching the relocation model on the subtarget, this is for
two reasons:

a) we're already caching the target machine which contains it,
b) which relocation model you get is dependent upon whether or
not you ask before MCCodeGenInfo is constructed on the target
machine, so avoid any latent issues there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213420 91177308-0d34-0410-b5e6-96231b3b80d8