C11 quick_exit() support for <stdlib.h>

C11 quick_exit() support for <stdlib.h>

Import some <stdlib.h> function declarations from latest FreeBSD and
implement them.  I am not sure if we should call the global reent
cleanup in quick_exit() similar to exit().

newlib/ChangeLog
2015-10-14  Sebastian Huber  <sebastian.huber@embedded-brains.de>

	* libc/include/stdlib.h (at_quick_exit): Declare.
	(quick_exit): Likewise.
	* libc/stdlib/Makefile.am (GENERAL_SOURCES): Add
	quick_exit.c.
	* libc/stdlib/Makefile.in: Regenerate.
	* libc/stdlib/quick_exit.c: New.