devel/llvm{19,20,21}: fix build on riscv64
<phase: check-sanity >
> You must select one and only one option from the BACKENDS single
> No option was selected (and one must be)
Approved by: portmgr (build fix blanket)
MFH: 2025Q3
(cherry picked from commit 39a32a2841cdf2ba43fcce1d8446abe1057ba755)