Add reference types R32 and R64

-Add resumable_trap, safepoint, isnull, and null instructions
-Add Stackmap struct and StackmapSink trait

Co-authored-by: Mir Ahmed <mirahmed753@gmail.com>
Co-authored-by: Dan Gohman <sunfish@mozilla.com>

cranelift/codegen/src/isa/riscv/binemit.rs

@@ -2,7 +2,7 @@
 
 use crate::binemit::{bad_encoding, CodeSink, Reloc};
 use crate::ir::{Function, Inst, InstructionData};
-use crate::isa::{RegUnit, StackBaseMask, StackRef};
+use crate::isa::{RegUnit, StackBaseMask, StackRef, TargetIsa};
 use crate::predicates::is_signed_int;
 use crate::regalloc::RegDiversions;
 use core::u32;