riscv/barrier: Consolidate fence definitions

riscv/barrier: Consolidate fence definitions

Disparate fence implementations are consolidated into fence.h.
Also introduce RISCV_FENCE_ASM to make fence macro more reusable.

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/20240217131316.3668927-1-ericchancf@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>