riscv: fix riscv64sf build

The library needs to have sf suffix cut off.

MFC after: 3 days
X-MFC with: 3781e77995e9b81afcbf7b3dc84b33b864c48794
Differential revision: https://reviews.freebsd.org/D34561
Reviewed by: dim

(cherry picked from commit 18f71c9b2779999a9492b42c325524651a1b6909)