Make le64 DescriptionString consistent with other targets.

Summary:
In particular, remove the defaults and reorder fields so it matches the result of DataLayout::getStringDescription().

Change by David Neto.

Reviewers: dschuff, sdt

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6482

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223140 91177308-0d34-0410-b5e6-96231b3b80d8