riscv: vdso: do not strip debugging info for vdso.so.dbg

The vdso.so.dbg is a debug version of vdso and could be used for debugging
purpose. For example, perf-annotate requires debugging info to show source
lines. So let's keep its debugging info.

Signed-off-by: Changbin Du <changbin.du@huawei.com>
Reviewed-by: Cyril Bur <cyrilbur@tenstorrent.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240611040947.3024710-1-changbin.du@huawei.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>