ld.so: Consolidate 2 strtouls into _dl_strtoul [BZ #21528]

ld.so: Consolidate 2 strtouls into _dl_strtoul [BZ #21528]

There are 2 minimal strtoul implementations in ld.so:

1. __strtoul_internal in elf/dl-minimal.c.
2. tunables_strtoul in elf/dl-tunables.c.

This patch adds _dl_strtoul to replace them.  Tested builds with and
without --enable-tunables.

	[BZ #21528]
	* elf/dl-minimal.c (__strtoul_internal): Removed.
	(strtoul): Likewise.
	* elf/dl-misc.c (_dl_strtoul): New function.
	* elf/dl-tunables.c (tunables_strtoul): Removed.
	(tunable_initialize): Replace tunables_strtoul with _dl_strtoul.
	* elf/rtld.c (process_envvars): Likewise.
	* sysdeps/unix/sysv/linux/dl-librecon.h (_dl_osversion_init):
	Likewise.
	* sysdeps/generic/ldsodefs.h (_dl_strtoul): New prototype.