compat: add a compat_need_64bit_alignment_fixup() helper

Add a helper to check if the calling syscall needs a fixup for
non-natural 64-bit type alignment in the compat ABI.  This will only
return true for i386 syscalls on x86_64.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>