Use libc_hidden_* for atoi (bug 15105).

Continuing the fixes for localplt test failures with -Os arising from
functions not being inlined in that case, this patch fixes such
failures for atoi by using libc_hidden_proto and libc_hidden_def.

Tested for x86_64 (both that it removes this particular localplt
failure for -Os, and that the testsuite continues to pass without
-Os).

	[BZ #15105]
	* stdlib/atoi.c (atoi): Use libc_hidden_def.
	* include/stdlib.h [!_ISOMAC] (atoi): Use libc_hidden_proto.