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.

(cherry picked from commit 20602c72fa)

include/stdlib.h

@@ -210,6 +210,8 @@ libc_hidden_proto (strtoll)
 libc_hidden_proto (strtoul)
 libc_hidden_proto (strtoull)
 
+libc_hidden_proto (atoi)
+
 extern float __strtof_nan (const char *, char **, char);
 extern double __strtod_nan (const char *, char **, char);
 extern long double __strtold_nan (const char *, char **, char);

stdlib/atoi.c

@@ -26,3 +26,4 @@ atoi (const char *nptr)
 {
   return (int) strtol (nptr, (char **) NULL, 10);
 }
+libc_hidden_def (atoi)