Ignore and remove LD_HWCAP_MASK for AT_SECURE programs (bug #21209)

Ignore and remove LD_HWCAP_MASK for AT_SECURE programs (bug #21209)

The LD_HWCAP_MASK environment variable may alter the selection of
function variants for some architectures.  For AT_SECURE process it
means that if an outdated routine has a bug that would otherwise not
affect newer platforms by default, LD_HWCAP_MASK will allow that bug
to be exploited.

To be on the safe side, ignore and disable LD_HWCAP_MASK for setuid
binaries.

	[BZ #21209]
	* elf/rtld.c (process_envvars): Ignore LD_HWCAP_MASK for
	AT_SECURE processes.
	* sysdeps/generic/unsecvars.h: Add LD_HWCAP_MASK.

(cherry picked from commit 1c1243b6fc33c029488add276e56570a07803bfd)