Add .gnu_debuglink when stripping DSOs.

This is used by debug tools to find symbols for the stripped binary.
Adds ~70 bytes to each DSO, ~25000 bytes total, which is about 0.01% of the
system image.

This change only affects -eng and -userdebug builds, and does not affect -user.

Change-Id: I0c92a9e09fbc97184dea66813923568cbcc2a3fe