Correctly detect major version of GCC (#20059)

We were doing a very poor job detecting the major version of GCC by
parsing the output of --version.

This patches uses -dumpversion to make this parsing straightforward and
it also fixes a bunch of compiling issues on different platforms with
custom output for --version switches. For example, openSUSE first line
of the output includes the revision number and the parsing that was
being done did mix that number with the major version and breaks
building the nim compiler (as it doesn't find the 3 dots for an X.Y.Z semver
format, hence returning "false").

In this patch, we simply use -dumpversion (which has been at least from
1993, so we are safe :)

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