util/cpuinfo-riscv: Use linux __riscv_hwprobe syscall

util/cpuinfo-riscv: Use linux __riscv_hwprobe syscall

With recent linux kernels, there is a syscall to probe for various
ISA extensions.  These bits were phased in over several kernel
releases, so we still require checks for symbol availability.

Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>