libgcc_s: expose __divmoddi4 on i386

GCC has used this for some time (since 7.0) and apparently we were
getting away with using the hidden symbol, but when linking with
--no-undefined-version we get an error unless it's properly exported.
(For anyone who wonders at the assymetry, __udivmoddi4 is indeed much
older and was introduced with GCC 3.0.)

MFC after: 3 days
Reviewed by: dim
Differential Revision: https://reviews.freebsd.org/D44878