native: add single-core mpe

am/arch/native/src/mpe.c

@@ -0,0 +1,21 @@
+#include <am.h>
+
+int _mpe_init(void (*entry)()) {
+  entry();
+  return 1;
+}
+
+int _ncpu() {
+  return 1;
+}
+
+int _cpu() {
+  return 0;
+}
+
+intptr_t _atomic_xchg(volatile intptr_t *addr, intptr_t newval) {
+  intptr_t result;
+  asm volatile ("lock xchg %0, %1":
+    "+m"(*addr), "=a"(result) : "1"(newval) : "cc");
+  return result;
+}