include/qemu/int128.h: Add function to create Int128 from int64_t

include/qemu/int128.h: Add function to create Int128 from int64_t

int128_make64() creates an Int128 from an unsigned 64 bit value; add
a function int128_makes64() creating an Int128 from a signed 64 bit
value.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210614151007.4545-34-peter.maydell@linaro.org