'make vdso_install' installs debug vdso files to /lib/modules/*/vdso/.

Only for the compat vdso on riscv, the installation destination differs;
compat_vdso.so.dbg is installed to /lib/module/*/compat_vdso/.

To follow the standard install destination and simplify the vdso_install
logic, change the install destination to standard /lib/modules/*/vdso/.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20231117125807.1058477-1-masahiroy@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>