riscv/barrier: Define __{mb,rmb,wmb}

Introduce __{mb,rmb,wmb}, and rely on the generic definitions for
{mb,rmb,wmb}. Although KCSAN is not supported yet, the definitions can
be made more consistent with generic instrumentation. Also add a space
to make the changes pass check by checkpatch.pl.
Without the space, the error message is as below:
ERROR: space required after that ',' (ctx:VxV)
26: FILE: arch/riscv/include/asm/barrier.h:23:
+#define __mb()         RISCV_FENCE(iorw,iorw)
                                        ^

Signed-off-by: Eric Chan <ericchancf@google.com>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20240217131249.3668103-1-ericchancf@google.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>