Even though this almost certainly wouldn't run properly even if we had
a working kernel for AARCH64 this at least lets us build all the
userland binaries.
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.
See #10042 for details. In short: qemu doesn't seem to implement that
feature, therefore the test correctly fails. However, that does not help
us, so we skip that test.
