riscv: isr.S: optimize FP regs save/restore decision

Rely on mstatus rather than thread->base.user_options since it is always
up to date (updated by z_riscv_switch) to simplify the code and be SMP
proof. Also carry over SF_INIT to the mstatus being restored in case
it was changed in the mean time.

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