x86: Properly set usable CET feature bits [BZ #26625]

commit 94cd37eb
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed Sep 16 05:27:32 2020 -0700

    x86: Use HAS_CPU_FEATURE with IBT and SHSTK [BZ #26625]

broke

GLIBC_TUNABLES=glibc.cpu.hwcaps=-IBT,-SHSTK

since it can no longer disable IBT nor SHSTK.  Handle IBT and SHSTK with:

1. Revert commit 94cd37eb.
2. Clears the usable CET feature bits if kernel doesn't support CET.
3. Add GLIBC_TUNABLES tests without dlopen.
4. Add tests to verify that CPU_FEATURE_USABLE on IBT and SHSTK matches
_get_ssp.
5. Update GLIBC_TUNABLES tests with dlopen to verify that CET is disabled
with GLIBC_TUNABLES.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>