graphics/libglvnd: allow the port to build on RISC-V
Use the same treatment as on PowerPC:
Program python3 found: YES (/usr/local/bin/python3.8) Program nm found: YES (/usr/bin/nm) Message: Host CPU family: riscv64 Message: Host CPU: riscv64 meson.build:82:4: ERROR: Problem encountered: No ASM available for freebsd (little endian)