include/qemu/int128.h: introduce bswap128s

include/qemu/int128.h: introduce bswap128s

Changes the current bswap128 implementation to use __builtin_bswap128
when available, adds a bswap128 implementation for !CONFIG_INT128
builds, and introduces bswap128s based on bswap128.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20210826145656.2507213-2-matheus.ferst@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>