kernel-devsrc: Add vdso.lds and other build files for riscv64 as well

These additional bits are needed on riscv64 as well
Fixes
make[1]: *** No rule to make target 'arch/riscv/kernel/vdso/vdso.lds', needed by 'arch/riscv/kernel/vdso/vdso.so.dbg'.  Stop.
make: *** [arch/riscv/Makefile:114: vdso_prepare] Error 2

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>