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   Implement PT_GETXSTATE and PT_SETXSTATE

   Introduce two new ptrace() requests: PT_GETXSTATE and PT_SETXSTATE,
   that provide access to the extended (and extensible) set of FPU
   registers on amd64 and i386.  At the moment, this covers AVX (YMM)
   and AVX-512 (ZMM, opmask) registers.  It can be easily extended
   to cover further register types without breaking backwards
   compatibility.

   PT_GETXSTATE issues the XSAVE instruction with all kernel-supported
   extended components enabled.  The data is copied into 'struct xstate'
   (which -- unlike the XSAVE area itself -- has stable format
   and offsets).

   PT_SETXSTATE issues the XRSTOR instruction to restore the register
   values from user-provided 'struct xstate'.  The function replaces only
   the specific XSAVE components that are listed in 'xs_rfbm' field,
   making it possible to issue partial updates.

   Both syscalls take a 'struct iovec' pointer rather than a direct
   argument.  This requires the caller to explicitly specify the buffer
   size.  As a result, existing code will continue to work correctly
   when the structure is extended (performing partial reads/updates).

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