riscv: FPU trap: catch fused multiply-add instructions

The FMADD, FMSUB, FNMSUB and FNMADD instructions occupy major opcode
spaces of their own, separate from LOAD-FP/STORE-FP and OP-FP spaces.
Insert code to cover them.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>