Make __tunables_init hidden and avoid PLT

Since __tunables_init is internal to ld.so, we should mark it hidden
to avoid PLT.  We should also avoid PLT when calling __tunable_set_val
within ld.so.

2017-05-25   Siddhesh Poyarekar  <siddhesh@sourceware.org>
	     H.J. Lu  <hongjiu.lu@intel.com>

	* elf/dl-tunables.c (__tunable_set_val): Make a hidden alias.
	* elf/dl-tunables.h (__tunables_init): Mark it hidden in rtld.
	(__tunable_set_val): Likewise.