Mark __libc_multiple_libcs with attribute_hidden [BZ #18822]

Since __libc_multiple_libcs is defined as hidden symbol in init-first.c,
it should be always marked with attribute_hidden.

	[BZ #18822]
	* csu/libc-start.c (__libc_multiple_libcs): Removed.
	* elf/dl-open.c: Include <libc-internal.h>.
	(__libc_multiple_libcs): Removed.
	* elf/dl-sysdep.c: Include <libc-internal.h> instead of
	<hp-timing.h>.
	* include/libc-internal.h (__libc_multiple_libcs): New.
	* misc/sbrk.c: Include <libc-internal.h>.
	(__libc_multiple_libcs): Removed.