linux: Use relative wrapper name and LD_LIBRARY_PATH

This allows the dynamic linker to load whatever wrapper DSO is appropriate,
making tracing on multiarch system much easier.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>

I'd guess that you'll want this reworked; this code works, but might not be of a style you find appropriate. I'll be happy to address your reviews.