Add NaCl to long double/fp128 mangling test

Summary:
NaCl is a platform where long double is the same as double.
Its mangling is spelled with "long double" but its ABI lowering is the same
as double.

Reviewers: rnk, chh

Subscribers: jfb, cfe-commits, dschuff

Differential Revision: http://reviews.llvm.org/D11922

llvm-svn: 244541