Avoid use of "register" as optimization hint

[David Miller]

From: Roland McGrath <roland@hack.frob.com>
Date: Fri,  7 Jun 2013 14:58:03 -0700 (PDT)

>> * Code from GMP, which also has lots of uses of "register".  I don't
>>   know if GMP upstream has since cleaned them up; we don't actually
>>   try to stay in sync with GMP upstream (updating GMP code from there
>>   would require FSF permission to use the current GMP versions of the
>>   relevant code under LGPLv2.1+).
> 
> I think the permission granted way back when applies just as well to newer
> GMP versions.  If you have a particular reason to think it wouldn't, then
> you should ask the appropriate FSF contact.  But I'm confident enough to
> just do whatever we want with FSF-owned code from recent GMP versions.

This topic came up when I wanted to use more recent GMP sparc code in
glibc's sysdeps, and I ended up only being able to use code that I had
written completely myself.

Others, at the time, did not express the same level of confidence you do.