cranelift-codegen: Add support for immediate to memory moves in x64 (#5461)

This change adds support for immediate to memory moves in x64 which
are needed by Winch for zeroing local slots.

This change follows the guideline in `isa/x64/inst/emit` and uses
other instructions (immediate to register moves) as a base for the
test cases.

The instruction encoding expectation was derived by assembling each
instruction and inspecting the assembly with `objdump`.

cranelift/codegen/src/isa/x64/inst.isle

@@ -126,6 +126,11 @@
                  (src GprMem)
                  (dst WritableGpr))
 
+       ;; Immediate store.
+       (MovImmM (size OperandSize)
+		(simm64 u64)
+		(dst SyntheticAmode))
+       
        ;; Integer stores: mov (b w l q) reg addr.
        (MovRM (size OperandSize) ;; 1, 2, 4, or 8
               (src Gpr)