Move emit and emit_safepoint to prelude.isle

Even though the implementation of emit and emit_safepoint may
be platform-specific, the interface ought to be common so that
other code in prelude.isle may safely call these constructors.

This patch moves the definition of emit (from all platforms)
and emit_safepoint (s390x only) to prelude.isle.  This required
adding an emit_safepoint implementation to aarch64 and x64 as
well - the latter is still a stub as special move mitosis
handling will be required.

cranelift/codegen/src/isa/aarch64/lower/isle.rs

@@ -203,6 +203,10 @@ where
         self.emitted_insts.push((inst.clone(), false));
     }
 
+    fn emit_safepoint(&mut self, inst: &MInst) -> Unit {
+        self.emitted_insts.push((inst.clone(), true));
+    }
+
     fn cond_br_zero(&mut self, reg: Reg) -> CondBrKind {
         CondBrKind::Zero(reg)
     }

cranelift/codegen/src/isa/aarch64/lower/isle/generated_code.manifest

@@ -1,4 +1,4 @@
 src/clif.isle 9ea75a6f790b5c03
-src/prelude.isle 51d2aef2566c1c96
-src/isa/aarch64/inst.isle f946561093de4ff5
+src/prelude.isle 2bfcafbef6b29358
+src/isa/aarch64/inst.isle 944323ff7d6db098
 src/isa/aarch64/lower.isle 2d2e1e076a0c8a23