riscv, bpf: Add kfunc support for RV64

This patch adds kernel function call support for RV64. Since the offset
from RV64 kernel and module functions to bpf programs is almost within
the range of s32, the current infrastructure of RV64 is already
sufficient for kfunc, so let's turn it on.
Suggested-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230221140656.3480496-1-pulehui@huaweicloud.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

arch/riscv/net/bpf_jit_comp64.c

@@ -1751,3 +1751,8 @@ void bpf_jit_build_epilogue(struct rv_jit_context *ctx)
 {
 	__build_epilogue(false, ctx);
 }
+
+bool bpf_jit_supports_kfunc_call(void)
+{
+	return true;
+}