Avoid use of "register" as optimization hint

[David Miller]

From: Roland McGrath <roland@hack.frob.com>
Date: Fri,  7 Jun 2013 15:12:08 -0700 (PDT)

>> 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.
> 
> Do you mean just others here, or did you ask a GMP maintainer and/or an FSF
> licensing representative?

Others here, who pointed this issue out when I posted the patch at the time.
For example:

http://sourceware.org/ml/libc-alpha/2013-01/msg00425.html

Which suggested we need to ask the FSF for approval.