[PATCH] RISC-V: don't assume PI mutexes and robust futexes before 4.20 (bug 23864)

[DJ Delorie]

Andreas Schwab <schwab@suse.de> writes:
> Support for futex_cmpxchg as only been added to 4.20-rc1.

I confirm it's not in 4.19.1.  I think it's safe to assume it will me in
4.20 official, so +1 from me.

Reviewed-by: DJ Delorie <dj@redhat.com>