Outdated Ubuntu build environment documentation

[Volodymyr Medvid]

How-to-build-toolchain.pdf provided with gcc-arm-none-eabi-10-2020-q4-major-src.tar.bz2 defines Ubuntu 14.04.5 as host system to build the toolchain for Linux and Windows targets.

The windows toolchain binaries produced from Ubuntu 14.04 host are affected by https://bugs.launchpad.net/gcc-arm-embedded/+bug/1848002 issue.

According to https://bugs.launchpad.net/gcc-arm-embedded/+bug/1848002/comments/5 comment, upgrading the build machine to ubuntu 18.04 resolves this problem.

I was able to confirm this on vanilla Ubuntu 18.04 virtual machine configured according to instructions from "1.2.1 Install dependencies available in Ubuntu's repositories" but with xenial/trusty specific hacks omitted.

Please update How-to-build-toolchain.pdf to document the recommended host system.