[RISCV] Add isel support for scalar STRICT_FADD/FSUB/FMUL/FDIV/FSQRT.

Test that STRICT_FMINNUM/FMAXNUM are lowered to libcalls for f32/f64.
The RISC-V instructions don't match the behavior of fmin/fmax libcalls
with respect to SNaN.

Promoting FMINNUM/FMAXNUM for f16 needs more work outside of the
RISC-V backend.

Reviewed By: asb, arcbbb

Differential Revision: https://reviews.llvm.org/D115680