[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.

llvm-svn: 215255