[ARM] Support constant pools in data when generating execute-only code.

The ARM backend asserts against constant pool lowering when it generates
execute-only code in order to prevent the generation of constant pools in
the text section. It appears that target independent optimizations might
generate DAG nodes that represent constant pools. By lowering such nodes
as global addresses we don't violate the semantics of execute-only code
and also it is guaranteed that execute-only behaves correct with the
position-independent addressing modes that support execute-only code.

Differential Revision: https://reviews.llvm.org/D33773

llvm-svn: 305387