x86/xsave: Make it clear that the XSAVE macros use (%edi)/(%rdi)

The XSAVE instruction family takes a memory argment.  The macros use
(%edi)/(%rdi) as that memory argument - make that clear to the reader.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1401387164-43416-7-git-send-email-fenghua.yu@intel.com