xchg() has up until now simply returned the x parameter in cases where
it is called with a pointer to a value of an unsupported size. This will
often cause the calling code to hit a failure path, presuming that the
value of x differs from the content of the memory pointed at by ptr, but
we can do better by producing a compile-time or link-time error such
that unsupported calls to xchg() are detectable earlier than runtime.

This patch does this in the same was as is already done for cmpxchg(),
using a call to a missing function annotated with __compiletime_error().
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16351/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>