TargetFrameLowering contains a callback for telling LLVM that a target
will do its own stack frame size rounding. This defaults to false.
In the RISC-V backend, we have been doing our own stack frame size rounding,
without overriding this callback. This has been leading to over-allocation,
which this patch aims to mitigate. In almost all cases of aligned objects on the
stack, stack usage has decreased.