Fix gen_getclassvariable

We need to reconstruct interpreter state before calling into the
routines to be able to raise exceptions. I'm getting a crash in
debug build with:
    make test-all 'TESTS=test/ruby/variable.rb' RUN_OPTS='--yjit-call-threshold=1 --yjit-max-versions=1'

yjit_codegen.c

@@ -4119,6 +4119,9 @@ rb_vm_get_cref(const VALUE *ep);
 static codegen_status_t
 gen_getclassvariable(jitstate_t* jit, ctx_t* ctx, codeblock_t* cb)
 {
+    // rb_vm_getclassvariable can raise exceptions.
+    jit_prepare_routine_call(jit, ctx, REG0);
+
     mov(cb, C_ARG_REGS[0], member_opnd(REG_CFP, rb_control_frame_t, ep));
     call_ptr(cb, REG0, (void *)rb_vm_get_cref);