The main change is making the scripts a two-stage process where we
first build GCC, leave the user to install the libc, and then come back
to install libstdc++-v3.
* Detect whether we are in the first or second stage
* Don't clean files after first stage install, and even then only do it
if :clean is specified
* Update README except for the manual install tutorial
* Patch the source using a backported GCC 12.1 commit to make the
configure script for libstdc++-v3 skip checking for dlopen, which is
impossible is our not-really-hosted setup
Details:
* Proper .gitignore
