Avoid undefined behaviour in ibm128 implementation of llroundl (BZ #29488)

Detecting an overflow edge case depended on signed overflow of a long
long. Replace the additions and the overflow checks by
__builtin_add_overflow().

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>
(cherry picked from commit 2b547856)