Turn off '-Wtransparent-union' by default

Turn off '-Wtransparent-union' by default

It's a very annoying warning, and it's about a sparse limitation rather
than a real feature, so don't do it by default.

Sure, our lack of transparent union support will then make us warn about
the magic calling convention hacks (eg you'll see warnings like

	connect.c:240:39: warning: incorrect type in argument 2 (invalid types)
	connect.c:240:39:    expected union __CONST_SOCKADDR_ARG [usertype] __addr
	connect.c:240:39:    got struct sockaddr *ai_addr

but it still doesn't mean that we have to be so noisy about just seeing
those transparent unions.  One annoying warning is not an argument for
doing _another_ annoying warning too (and the calling convention
warnings at least only happen if you actually use them, unlike the
transparent union warning that happens every time we see one, used or
not).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christopher Li <sparse@chrisli.org>