ARM: fix multiprecision adds and subtracts.

The former code used an adds/adc or subs/sbc pair, which works fine,
except that it neglected to declare it as clobbering the condition
codes.  Since it does (and kinda has to, since the carry bit is used
to, well, handle the carry), this is broken - and, at -O2 (and possibly
others) led to broken code because the optimizer moved the loop control
instruction across an asm that it had been told didn't mess with the
condition codes, leading to loops not looping correctly.

So, we just tell gcc that the sequence clobbers the condition codes and
it works.