[MCJIT] Simplify immediate decoding code in the RuntimeDyldMachO hierarchy.

[MCJIT] Simplify immediate decoding code in the RuntimeDyldMachO hierarchy.

Cleanup only: no functional change.

This patch makes RuntimeDyldMachO targets directly responsible for decoding
immediates, rather than letting them implement catch a callback from generic
code. Since this is a very target specific operation, it makes sense to let the
target-specific code drive it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215255 91177308-0d34-0410-b5e6-96231b3b80d8