[ARM] Generate ABI_optimization_goals build attribute, as described in the ARM ARM.

Summary:
Since this build attribute corresponds to a whole module, and
different functions in a module may differ in the optimizations
enabled for them, this attribute is emitted after all functions,
and only in the case that the optimization goals for all
functions match.

Reviewers: logan, hans

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D14934

llvm-svn: 254201