math/R: Update to version 3.4.1

Upstream changes:
https://cran.r-project.org/doc/manuals/r-release/NEWS.html

Port changes:

• Add aarch64 to ONLY_FOR_ARCHS [1]
• Add off-by-default option for experimental fortran support via flang [2]

Submitted by: [1] andrew, [2] yuri@rawbw.com