baremetal: define the abort function

This is normally provided by the libc, but in our case it makes more
sense to define it in the runtime (just like malloc).

src/runtime/baremetal.go

@@ -46,6 +46,11 @@ func libc_free(ptr unsafe.Pointer) {
  free(ptr)
 }
 
+//export abort
+func libc_abort() {
+ abort()
+}
+
 //go:linkname syscall_Exit syscall.Exit
 func syscall_Exit(code int) {
  abort()