Mark as BROKEN on riscv64:

configure:26760: checking for /nxb-bin/usr/bin/c++
configure:26788: result: no
configure:26797: error: Easybindings require a C++ compiler, but none were found.

NB: in my jail, /nxb-bin/usr/bin/c++ does indeed exist.

Approved by: portmgr (tier-2 blanket)
Obtained from: lonesome.com build testing