Add new variable 'inhibit-native-compilation'

Add new variable 'inhibit-native-compilation'

* doc/lispref/compile.texi (Native-Compilation Variables):
Document it.

* lisp/startup.el (normal-top-level): Set
inhibit-native-compilation from environment variable.

* lisp/subr.el (native-comp-deferred-compilation): Make obsolete.

* lisp/emacs-lisp/comp.el (comp-trampoline-compile): Don't write
trampolines to disk.

* lisp/progmodes/elisp-mode.el
(emacs-lisp-native-compile-and-load): Adjust.

* src/comp.c (syms_of_comp): New variable
inhibit-native-compilation.
(maybe_defer_native_compilation): Use it.