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

commit 04bba1e5
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed Aug 5 13:51:56 2020 -0700

    x86: Set CPU usable feature bits conservatively [BZ #26552]

    Set CPU usable feature bits only for CPU features which are usable in
    user space and whose usability can be detected from user space, excluding
    features like FSGSBASE whose enable bit can only be checked in the kernel.

no longer turns on the usable bits of IBT and SHSTK since we don't know
if IBT and SHSTK are usable until much later.  Use HAS_CPU_FEATURE to
check if the processor supports IBT and SHSTK.