tcg: Combine dh_is_64bit and dh_is_signed to dh_typecode

tcg: Combine dh_is_64bit and dh_is_signed to dh_typecode

We will shortly be interested in distinguishing pointers
from integers in the helper's declaration, as well as a
true void return.  We currently have two parallel 1 bit
fields; merge them and expand to a 3 bit field.

Our current maximum is 7 helper arguments, plus the return
makes 8 * 3 = 24 bits used within the uint32_t typemask.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>