riscv: Separate toolchain support dependency from RISCV_ISA_ZACAS

RV64 bpf is going to support ZACAS instructions. Let's separate
toolchain support dependency from RISCV_ISA_ZACAS.

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/20250719091730.2660197-5-pulehui@huaweicloud.com