arch/arm64: z_fatal_error() can return!

This isn't a FUNC_NORETURN function.  On a handled
k_sys_fatal_error_handler(), we'll end up back here.  And the
CODE_UNREACHABLE was causing code generateion bugs that prevented
return up the stack.

Note that the same pattern exists in MANY other architectures (it
seems like the original was part of x86).  But in most cases the arch
assembly isn't actually capable of handling the return to an aborted
thread frame correctly.  On arm64, it does actually work.  So fix it
here at least.

Signed-off-by: Andy Ross <andyross@google.com>