tools: Add riscv barrier implementation

tools: Add riscv barrier implementation

Many of the other architectures use their custom barrier implementations.
Use the barrier code from the kernel sources to optimize barriers in
tools.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20240806-optimize_ring_buffer_read_riscv-v2-1-ca7e193ae198@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>