math/R: Update to version 3.4.3

Other port changes:

• Add armv6 and armv7 to ONLY_FOR_ARCHS; armv6 and armv7 were tested by mmel@ and working on 11.
• Remove LIBR option and always build the shared library. This was requested by yuri@ and a RFC was posted on ports@ with no response.
• Exclude OPENMP option for everything except amd64 and i386.
• Turn on OPENMP option by default to match upstream.
• Turn off TCLTK option by default.
• LLVM, which gets pulled in with FLANG, provides libomp, so only depend on devel/openmp, when GFORTRAN and OPENMP are on.