[macho] Allow CPUSubtype to be adjusted before writing it to a file

Summary:
It's not always possible for a target to provide a MachO CPUSubtype up
front as is required by the current implementation. Sometimes you need more
information like the particular CPU implementation you are targeting.

Give MCMachObjectTargetWriter subclasses the opportunity to modify the
CPUSubtype after the MCMachObjectTargetWriter is created but before the
object starts being written. Typically this would be done in response to
instructions from a TargetStreamer.

Reviewers: pete

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70344