cmd/internal/obj/riscv: handle FEQ/FNEG/SEQZ/SNEZ

Based on riscv-go port.

Updates #27532

Change-Id: I5e7f45955e1dfdb9d09cc6a4e6f3ce81216d411d
Reviewed-on: https://go-review.googlesource.com/c/go/+/204628


Reviewed-by: Cherry Zhang <cherryyz@google.com>