Add linux-x86_64 generic to current unadopted platforms

[rainerjung]

The list of current platforms under /general-policies/tree/master/policy-supplemental/platforms.md currently only contains one entry for Linux with x86_64 and gcc. It is under primary platforms, but explicitly mentions Ubuntu Server 20.04 and gcc 9.

I suggest to add a generic entry to the list of unadopted platforms, just like the one that is there for clang:
linux-x86_64-gcc Linux x86_64 gcc
Of course it would be nice to also add somewhere the info, what your ambition about supported gcc versions is.

Without this generic entry, your documented Linux x86_64 support with gcc is "just" Ubuntu Server 20.04 , independent of support level.

[nhorman]

Its probably worth adding a link to our platform policy here:
https://openssl-library.org/policies/general/platform-policy/

Based on that, we certainly could add a generic entry to the unadopted platform section, but I'm not sure what benefit that would have. It would just mean that a target exists (which linux_generic64 does), but that no effort is made to test or confirm that it works on a given os/toolchain. 99% of the time it works, because recent toolchains are largely invariant (or their variability is accounted for). but if you run into an issue in which something breaks, there is no guarantee of a fix.