Use libc_hidden_* for argz_next, __argz_next (bug 15105).

Among other localplt test failures when building with -Os, there are
libc.so PLT references for argz_next and __argz_next.  This is a
simple case of functions that are inlined for -O2 but not for -Os;
this patch adds libc_hidden_proto / libc_hidden_def for them to avoid
localplt failures even when not inlined.

Tested for x86_64 (both that it removes these particular localplt
failures for -Os - but other such failures remain so the bug can't yet
be closed - and that the testsuite continues to pass without -Os).

	[BZ #15105]
	* include/argz.h (argz_next): Use libc_hidden_proto.
	(__argz_next): Likewise.
	* string-argz-next.c (__argz_next): Use libc_hidden_def.
	(argz_next): Use libc_hidden_weak.

(cherry picked from commit 055ac2a7)