math/big: use more efficient way to save/restore CF flag #9243
Description
From personal e-mail with Torbjörn Granlund:
"About saving/restoring the CF flag:
RCRQ/RCLQ are really slow on some archs, and slow on other. It is
better to use common insns like SBB Rx,Rx for saving and ADD Rx,Rx
for restoring."