If the CPU cores implement FEAT_IDST, emulate access to the CPU ID

If the CPU cores implement FEAT_IDST, emulate access to the CPU ID
registers from userland and set HWCAP_CPUID.  This will allow detection
of features to be introduced into the architecture in the future without
allocating new HWCAP_xxx or HWCAP2_xxx bits.  We provide the same
sanitized view of the CPU ID registers as is currently available through
sysctl(2).

Note that this introduces an unconditional read of ID_AA64MMFR2_EL1.  This
is known to cause problems on older versions of QEMU.  If this turns out
to be a problem in cases where updating QEMU is not an option, we'll have
to implement a workaround.

Also note that since we don't emulate the CPU ID registers on older core,
this means that microarchitectural optimizations keyed of reads of MIDR_EL1
are not possible on OpenBSD.  I don't think that is a real problem.

ok jca@