access saved values of upper xmm/ymm/zmm registers in signal handler

[Florian Weimer]

* Joseph Devietti via Libc-help:

> I was wondering if there's a way to access the saved values of X86-64's
> second set of 16 vector registers (xmm16-31, ymm16-31 and/or zmm16-31)
> inside a SA_SIGINFO signal handler, via the ucontext_t* pointer. I've tried
> digging around the code and couldn't find anything promising, but maybe I
> missed something.

I think the date is in struct _xstate, which can be accessed via the
struct sigcontext and its fpstate member.  The format should follow
what the XSAVE* instructions produce; you will have to review the
processor manual to figure out how to decode it.