libcxx: Make gen_link_script.py print contents only in --dryrun mode

The build should generally be quiet if there are no errors,
and this script has been around long enough that we can remove
the log output. If we ever need to debug something with this script,
we can put back the logging then.

Differential Revision: https://reviews.llvm.org/D66594

llvm-svn: 369757