riscv, bpf: Add rv_ext_enabled macro for runtime detection extentsion

Add rv_ext_enabled macro to check whether the runtime detection
extension is enabled.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/bpf/20250719091730.2660197-6-pulehui@huaweicloud.com