Enable Darwin vararg parameters support in assembler macros.

Duplicate the vararg tests for linux and add a tests which mixed
vararg arguments with darwin positional parameters.

Patch by: Janne Grunau <j@jannau.net>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214799 91177308-0d34-0410-b5e6-96231b3b80d8