bpf: Use the getSubtarget call off of the MachineFunction rather than the TargetMachine

Summary:
Hi Eric,

this patch cleans up the layering violation that you're fixing across backends.
Anything else I need to fix on bpf backend side?

Thanks

Reviewers: echristo

Reviewed By: echristo

Differential Revision: http://reviews.llvm.org/D7355

llvm-svn: 227865