math/sleef: Update to latest upstream commit (2021-04-05)

This updates sleef to latest upstream commit
85440a5e87dae36ca1b891de14bc83b441ae7c43 which fixes a bunch of things
including AVX2 detection. DFT library is now disabled by default because
it has issues indicated by issue reports in upstream repo. There are
also some issues with unit tests and/or library itself which may
results in bugs however unit tests works fine on 13.2-RELEASE amd64
at least. Import patch from Debian to fix build of unit tests

References:
https://github.com/shibatch/sleef/issues/439
https://salsa.debian.org/science-team/sleef/-/blob/master/debian/patches/disable-duplicate-mpfr-funcs.patch

PR: 266784
Reviewed by: jmd (maintainer)