Add NaCl (a target where long double = double) to long double ABI test

A test was recently (r244468) added to cover long double calling convention
codegen, distinguishing between Android and GNU conventions (where long doubles
are fp128 and x86_fp80, respectively). Native Client is a target where long
doubles are the same as doubles. This change augments the test to cover
that case.

Also rename the test to test/codeGen/X86_64-longdouble.c

Differential Revision: http://reviews.llvm.org/D11921

llvm-svn: 244524