Four day week (Happy Thanksgiving, all you Europeans)

[Upstream]

Posted patches making bswap during qemu_ld/st optional. The prompt was reviewing the tcg/riscv/ patch set, but I've wanted to do this for ages. This should avoid the need for reservation of scratch registers on some hosts that can ill afford it -- in particular i386 and aa32.

In the process I ran into two other places that x86 used those scratch registers: stores from non-q-registers and bsd-user.

I included patches for both of those (freebsd only; net/openbsd will work so long as guest_base == 0 and will abort at runtime otherwise).

I still need to audit the other hosts before dropping the call-clobbering from user-only within tcg.c.

r~