vdso: Pass along path prefix maps to the vdso-elf.so script

[markj]

Summary

We build elf-vdso.so.o with -g, so the object file contains build
metadata. To ensure that kernel builds are reproducible, we need to pass
along the prefix map parameters added in commit eebadc7d8590
("sys/conf: Remap paths for reproducibility").

[kib]

The space in the single shared page we use ATM is limited. I would prefer that we would strip unneeded stuff from vdso instead. Does this mean that we should remove '-g' from cc options?

[markj]

In D52129#1190536, @kib wrote:

The space in the single shared page we use ATM is limited. I would prefer that we would strip unneeded stuff from vdso instead. Does this mean that we should remove '-g' from cc options?

I think it is enough to simply remove ${DEBUG}. I will test it and see.

[markj]

Simply remove the debug flag instead