Inline assembly needs "cc" clobber to avoid miscompilation

[Torbjörn Granlund]

I used a few more hours of my time on this.

My conclusion is that "cc" clobber is completely ignored by clang.  In
fact, clang happily schedules carry-set and carry-use around inline asm
with "cc".

This is another example that it's just a waste time trying to make GMP
compile reliably with clang.  See also Table 3 and the list below that
table at https://gmplib.org/devel/tm/gmp/date.html for some context.

Perhaps we could help clang's users by disabling GMP's inline asm
altogether.  It make things run a bit slower, but correctness is not
optional, speed is.  (We never enabled inline asm, it got enabled since
clang claims to be gcc.)

-- 
Torbjörn
Please encrypt, key id 0xC8601622