Re-fix build warnings about subr-x defsubsts

Re-fix build warnings about subr-x defsubsts

* lisp/term/haiku-win.el (require):
* lisp/progmodes/elisp-mode.el (require):
* lisp/isearch.el (require): Require subr-x at compile time, since
these use defsubsts/macros from there.

* lisp/emacs-lisp/subr-x.el (string-empty-p): Move from here...
* lisp/simple.el (string-empty-p): ... to here.  This is to help
with a build problem where files.el is using the defsubst, but
requiring subr-x.el at compile time leads to load errors.