[ARM] Fix AAPCS-VFP non-compliance when returning HFA from variadic functions.

Arguments and return values must always be marshalled as for the base
AAPCS when the callee is a variadic function.

Patch by Oliver Stannard!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200307 91177308-0d34-0410-b5e6-96231b3b80d8