linux-user/loongarch64: Add vdso

Requires a relatively recent binutils version in order to avoid
spurious R_LARCH_NONE relocations.  The presence of these relocs
are diagnosed by our gen-vdso tool.

Tested-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>