sh: Fix up xchg/cmpxchg corruption with gUSA RB.

gUSA special cases r15 for part of its login/out sequence, meaning that
any parameters need to be explicitly prohibited from accidentally being
assigned that particular register, and the compiler ultimately needs to
use a temporary instead.

Certain configurations have begun generating code paths that do indeed
get allocated r15, resulting in immediate corruption of the exchanged
value. This was observed in (amongst others) exit_mm() code generation
where the xchg_u32 call was immediately corrupting a structure address.

As this is a general gUSA restriction, the rest of the users likewise
need to be updated to ensure sensible constraints.

References: https://bugzilla.stlinux.com/show_bug.cgi?id=11229


Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Reviewed-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>