openrisc: Union fpcsr and oldmask in sigcontext to unbreak userspace ABI

With commit 27267655 ("openrisc: Support floating point user api") I
added an entry to the struct sigcontext which caused an unwanted change
to the userspace ABI.

To fix this we use the previously unused oldmask field space for the
floating point fpcsr state.  We do this with a union to restore the ABI
back to the pre kernel v6.4 ABI and keep API compatibility.

This does mean if there is some code somewhere that is setting oldmask
in an OpenRISC specific userspace sighandler it would end up setting the
floating point register status, but I think it's unlikely as oldmask was
never functional before.

Fixes: 27267655 ("openrisc: Support floating point user api")
Reported-by: Szabolcs Nagy <nsz@port70.net>
Closes: https://lore.kernel.org/openrisc/20230626213840.GA1236108@port70.net/


Signed-off-by: Stafford Horne <shorne@gmail.com>