[libc] Fix float128-to-integer conversion UB  (#211593)

https://github.com/llvm/llvm-project/pull/200565#discussion_r3643991194
Fixes the recent issue pointed out in the above comment
about undefined behavior for float128 in the case of extremely large
values and extremely small values.

GitOrigin-RevId: f79a8888eb831238423362d004739683e2a20d93