riscv: Rename __switch_to_aux() -> fpu

The name of __switch_to_aux() is not clear and rename it with the
determine function: __switch_to_fpu(). Next we could add other regs'
switch.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Reviewed-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>