Use "load-prefer-newer" for loading Spacemacs core

[sunlin7]

Currently the Spacemacs try to remove all *.elc at the beginning of startup, and set load-prefer-newer in the layers/+spacemacs/spacemacs-defaults/config.el.

Here we set the load-prefer-newer to t during startup to do similar work; and Spacemacs will cleaup the staled *.elc in the after startup hook.

spacemacs/core/core-spacemacs.el

Lines 374 to 379 in c3269f6

	(if dotspacemacs-byte-compile
	(when (> 1 (spacemacs//dir-byte-compile-state
	(concat spacemacs-core-directory "libs/")))
	(byte-recompile-directory (concat spacemacs-core-directory "libs/") 0))
	(spacemacs//remove-byte-compiled-files-in-dir spacemacs-core-directory))