Make LD_HWCAP_MASK usable for static binaries

The LD_HWCAP_MASK environment variable was ignored in static binaries,
which is inconsistent with the behaviour of dynamically linked
binaries.  This seems to have been because of the inability of
ld_hwcap_mask being read early enough to influence anything but now
that it is in tunables, the mask is usable in static binaries as well.

This feature is important for aarch64, which relies on HWCAP_CPUID
being masked out to disable multiarch.  A sanity test on x86_64 shows
that there are no failures.  Likewise for aarch64.

	* elf/dl-hwcaps.h [HAVE_TUNABLES]: Always read hwcap_mask.
	* sysdeps/sparc/sparc32/dl-machine.h [HAVE_TUNABLES]:
	Likewise.
	* sysdeps/x86/cpu-features.c (init_cpu_features): Always set
	up hwcap and hwcap_mask.