Wrap getenv when linking a coverage-enabled binary

The wrapper to getenv() appends the effective userid (euid) of the
current process to GCOV_PREFIX.  This avoids conflicts and permissions
issues when multiple processes try to create/access the same directories
and files under /data/misc/trace.

Test: Verify that coverage files are written to
/data/misc/trace/<euid>/proc/... instead of /data/misc/trace/proc/...

Change-Id: If58081a12b2b9bc40cfcbf64c99beafe198d07af