New x86-64 micro-architecture levels

[Florian Weimer]

* Mark Wielaard:

> One thing that wasn't clear to me from this proposal is how the glibc
> dynamic loader checks for the CPU feature flags. This is important for
> valgrind since it can communicate those through different means. cpuid
> interception, auxv AT_HWCAP/AT_HWCAP2 interception (but not AT_PLATFORM
> at the moment) and of course we can generate SIGILL for unsupported
> instructions. We currently don't intercept /proc/cpuinfo (but could).

glibc uses CPUID in combination with XGETBV.  There is also a masking
feature which I have not reviewed, but given that it only takes features
away, I don't think it matters to valgrind.

Thanks,
Florian