Too low open files limit

If we are to link qtwebengine with the default BFD linker instead of the gold linker (see also https://codereview.qt-project.org/c/qt/qt5/+/327639), we need a higher limit for maximum open files. See the following error logs:

http://coin.intra.qt.io/coin/api/results/qt/qtwebengine/8631ee6ae8fda73f9635a0b395898041b965a60e/LinuxUbuntu_18_04x86_64LinuxUbuntu_18_04x86_64GCCqtci-linux-Ubuntu-18.04-x86_64-761459Documentation_OutOfSourceBuild/eb2965151f906ca49d90ae5cc04210d824566b0e/build_1609748989/log.txt.gz

and

http://coin.intra.qt.io/coin/api/results/qt/qtwebengine/8631ee6ae8fda73f9635a0b395898041b965a60e/LinuxUbuntu_18_04x86_64LinuxUbuntu_18_04x86_64GCCqtci-linux-Ubuntu-18.04-x86_64-761459/eb2965151f906ca49d90ae5cc04210d824566b0e/build_1609748984/log.txt.gz

This is reproducible locally. I also needed to raise the default maximum open files on my Linux installation to build with the old linker.