The filename of temporary files, especially the basename of C files, can
end up in the output in some form, e.g. as part of linker debug
information. In the interest of bit-wise exactly reproducible
compilation (#4012), the basename of the temporary file no longer
contains random information (it used to ontain the process id).

This is ok, as the temporary directory used contains the pid (see
getTempDir).

This patch has been applied to the Debian package (version 7.10.1-5) and
allowed a fully bit-wise reproducible build:
https://reproducible.debian.net/rb-pkg/experimental/amd64/ghc.html

Reviewed By: austin, rwbarton

Differential Revision: https://phabricator.haskell.org/D910

GHC Trac Issues: #4012