[XRay] [compiler-rt] Include argv[0] in the log file name.

Summary:
If you decide to recompile parts of your Linux distro with XRay, it may
be useful to know which trace belongs to which binary.  While there, get
rid of the incorrect strncat() usage; it always returns a pointer to the
start which makes that if() always true.  Replace with snprintf which is
bounded so that enough from both strings fits nicely.

Reviewers: dberris

Subscribers: danalbert, srhines, kubabrecka, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D27912

llvm-svn: 290861