Improve the detection of the path

Summary:
When clang is used under GNU/Linux in a chroot without /proc mount, it falls
back on the BSD method. However, since the buf variable is used twice
and fails with snprintf to produce the correct path.

When called as relatived (ie ./clang), it was failing with:
 "" -cc1 [...] -x c++ x.cc
error: unable to execute command: Executable "" doesn't exist!


I also took the opportunity to simply the code (the first arg of test_dir
was useless).

Reviewers: rafael

Reviewed By: rafael

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2361