linux-user/host/riscv: Improve host_signal_write

linux-user/host/riscv: Improve host_signal_write

Do not read 4 bytes before we determine the size of the insn.
Simplify triple switches in favor of checking major opcodes.
Include the missing cases of compact fsd and fsdsp.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>