bsd-user: Define TARGET_SIGSTACK_ALIGN and use it to round stack

bsd-user: Define TARGET_SIGSTACK_ALIGN and use it to round stack

Most (all?) targets require stacks to be properly aligned. Rather than a
series of ifdefs in bsd-user/signal.h, instead use a manditory #define
for all architectures.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>