clang/test/CodeGen/2008-01-25-ByValReadNone.c: Add explicit -triple x86_64-unknown-unknown.

It would emit @llvm.memcpy with "-triple x86_64-(mingw32|win32)" and had been failing since Nick's r185735.

  ; Function Attrs: nounwind
  declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly, i64, i32, i1) #1

llvm-svn: 185796