tunables: Use glibc.tune.hwcap_mask tunable instead of _dl_hwcap_mask

Drop _dl_hwcap_mask when building with tunables.  This completes the
transition of hwcap_mask reading from _dl_hwcap_mask to tunables.

	* elf/dl-hwcaps.h: New file.
	* elf/dl-hwcaps.c: Include it.
	(_dl_important_hwcaps)[HAVE_TUNABLES]: Read and update
	glibc.tune.hwcap_mask.
	* elf/dl-cache.c: Include dl-hwcaps.h.
	(_dl_load_cache_lookup)[HAVE_TUNABLES]: Read
	glibc.tune.hwcap_mask.
	* sysdeps/sparc/sparc32/dl-machine.h: Likewise.
	* elf/dl-support.c (_dl_hwcap2)[HAVE_TUNABLES]: Drop
	_dl_hwcap_mask.
	* elf/rtld.c (rtld_global_ro)[HAVE_TUNABLES]: Drop
	_dl_hwcap_mask.
	(process_envvars)[HAVE_TUNABLES]: Likewise.
	* sysdeps/generic/ldsodefs.h (rtld_global_ro)[HAVE_TUNABLES]:
	Likewise.
	* sysdeps/x86/cpu-features.c (init_cpu_features): Don't
	initialize dl_hwcap_mask when tunables are enabled.