tcg: Split rem requirement from div requirement

There are several hosts with only a "div" insn.  Remainder is computed
manually from the quotient and inputs.  We can do this generically.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-arm: Don't implement rem

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-ppc: Don't implement rem

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-ppc64: Don't implement rem

Signed-off-by: Richard Henderson <rth@twiddle.net>