microblaze: Fix __futex_atomic_op macro register usage

commit 8cf662ed3ef190fddc186bb5b1cd75eb3880d5a9 upstream.

Old Microblaze toolchain supported "b" contstrains for
all register but it always points to general purpose reg.
New Microblaze toolchain is more strict in this
and general purpose register should be used there "r".

Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>