x32. Fixes a bug in how struct va_list is initialized in x32

Summary: This patch modifies X86TargetLowering::LowerVASTART so that
struct va_list is initialized with 32 bit pointers in x32. It also
includes tests that call @llvm.va_start() for x32.

Patch by João Porto

Subscribers: llvm-commits, hjl.tools
Differential Revision: http://reviews.llvm.org/D12346

llvm-svn: 247069