In AArch CI, this test alone takes up 110.6 seconds. In x86_64 CI, it
takes up 68.4 seconds. There is no reason to spend this much time and
this many trials on this.
Let's reduce the number of iterations to 500. This should still surface
any misalignment with high probability, and should speed up the CI time
from minutes to seconds.
Doesn't use them in libc headers so that those don't have to pull in
AK/Platform.h.
AK_COMPILER_GCC is set _only_ for gcc, not for clang too. (__GNUC__ is
defined in clang builds as well.) Using AK_COMPILER_GCC simplifies
things some.
AK_COMPILER_CLANG isn't as much of a win, other than that it's
consistent with AK_COMPILER_GCC.
