AMDGPU/SI: Pass whether to use the SI scheduler via Target Attribute

Summary:
Currently the SI scheduler can be selected via command line option,
but it turned out it would be better if it was selectable via a Target Attribute.

This patch adds "si-scheduler" attribute to the backend.

Reviewers: tstellarAMD, echristo

Subscribers: echristo, arsenm

Differential Revision: http://reviews.llvm.org/D16192

llvm-svn: 258386