Add back support for accumulating outgoing arguments

The unwind rework (commit 2d5db92a) removed support for the
feature to allow a target to allocate the space for outgoing
function arguments right in the prologue (originally added
via commit 80c2d70d).   This patch adds it back.

cranelift/codegen/src/isa/x64/abi.rs

@@ -500,6 +500,7 @@ impl ABIMachineSpec for X64ABIMachineSpec {
         flags: &settings::Flags,
         clobbers: &Set<Writable<RealReg>>,
         fixed_frame_storage_size: u32,
+        _outgoing_args_size: u32,
     ) -> (u64, SmallVec<[Self::I; 16]>) {
         let mut insts = SmallVec::new();
         // Find all clobbered registers that are callee-save.
@@ -574,6 +575,7 @@ impl ABIMachineSpec for X64ABIMachineSpec {
         flags: &settings::Flags,
         clobbers: &Set<Writable<RealReg>>,
         fixed_frame_storage_size: u32,
+        _outgoing_args_size: u32,
     ) -> SmallVec<[Self::I; 16]> {
         let mut insts = SmallVec::new();