math/saga: update 6.3.0 -> 6.4.0, fix describe

• Change from llvm50 to llvm60
• Use OpenMP from within llvm60 instead of devel/openmp
• disable OpenMP, if i386 is detected

PR: 229470, 229129
Submitted by: Rainer Hurling <rhurlin@gwdg.de> (maintainer)
Reported by: Uasyhiro KIMURA <yasu@utahime.org>
Reviewed by: w.schwarzenfeld@utanet.at
Relnotes: https://sourceforge.net/p/saga-gis/news/2018/06/saga-640-released/