Implement them in softfloat and remove the local versions in riscv.

Signed-off-by: Kito Cheng <kito.cheng@sifive.com>
Signed-off-by: Chih-Min Chao <chihmin.chao@sifive.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1596102747-20226-2-git-send-email-chihmin.chao@sifive.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>