Also move osl/util.c on Unix systems to osl/system.c.

Change-Id: Ifff79d9f4f89ecbb4e0e1652b40ab46b7d569adf
Reviewed-on: https://gerrit.libreoffice.org/6065Tested-by: Arnaud Versini <arnaud.versini@libreoffice.org>
Reviewed-by: Arnaud Versini <arnaud.versini@libreoffice.org>