target/riscv: Split gen_arith_imm into functional and temp

target/riscv: Split gen_arith_imm into functional and temp

The tcg_gen_fooi_tl functions have some immediate constant
folding built in, which match up with some of the riscv asm
builtin macros, like mv and not.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>