[RISCV64] Emit correct lib call for fp(float/double) to ui/si

Since i32 is not legal in riscv64,
it always promoted to i64 before emitting lib call and
for conversions like float/double to int and float/double to unsigned int
wrong lib call was emitted. This commit fix it using custom lowering.

Differential Revision: https://reviews.llvm.org/D80526

(cherry picked from commit 7622ea58)