libsel4: Ensure the ARM syscall stubs always treat instances of seL4_MessageInfo as POD

A C++ compiler will attempt to interpret 'return info' as an invocation of a copy
constructor. Even though the copy constructor is auto-generated (and eventually
completely eliminated due to inlining) and just does obvious member copying it
still results in an intermediate invocation that must take a reference to 'info'.
Because 'info' is a register variable it is not permissable to take a reference
to it. Manually reconstructing a new info from the .words keeps everything as
Plain Old Data