[analyzer] SATestBuild: strip trailing newlines from command file.

When running a make-based command, SATestBuild tries to append a -jN flag
with an appropriate N to run the build in parallel. However, it failed
to take into account that each line read includes a trailing newline
(unless it is the last line of a file without a trailing newline), which
resulted in the "-jN" appearing on a line on its own.

llvm-svn: 190164