host-utils: add 128-bit quotient support to divu128/divs128

These will be used to implement new decimal floating point
instructions from Power ISA 3.1.

The remainder is now returned directly by divu128/divs128,
freeing up phigh to receive the high 64 bits of the quotient.

Signed-off-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211025191154.350831-4-luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>