lib: sbi: Implement SET_FS_DIRTY() to make sure the mstatus FS dirty is set

We found the mstatus.FS status is not set correctly after the SET_F64_REG()
and SET_F32_REG(). We should set mstatus.FS dirty after we emulate the FPU
instructions.

Co-developed-by: Roy Lin <roy.lin@sifive.com>
Signed-off-by: Roy Lin <roy.lin@sifive.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>