Get __STRICT_ALIGNMENT from <machine/endian.h> and decide upon it, rather

Get __STRICT_ALIGNMENT from <machine/endian.h> and decide upon it, rather
than defining it for not (i386 and amd64 (and sometimes s390)) only.

Compile-time tests remain compile-time tests, and runtime-test remain
runtime-test instead of being converted to compile-time tests, per matthew@'s
explicit demand (rationale: this makes sure the compiler checks your code even
if you won't run it).

No functional change except on s390 (which we don't run on) and vax (which we
run on, but noone cares about)

ok matthew@