m32r: __xchg() should be always_inline

it depends on elimination of unreachable branches in switch (by object
size), so we must declare it always_inline

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Hirokazu Takata <takata@linux-m32r.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>