linux/common-config: disable OF_OVERLAY on x86_64 if version < 5.15

`OF_OVERLAY` breaks kernel compilation for version 5.10
(beginning with 5.10.236), but only on x86_64.
Luckily, `OF_OVERLAY` is of not real interest on x86_64,
so this commit unbreaks the build by just disabling that option
if the target is x86_64 *and* the kernel version is below 5.15.

v5.15 contains a patch that turns the compilation error into a
warning, so this measure is not needed from that version on:
https://git.kernel.org/linus/5aad03685185b5133a28e1ee1d4e98d3fd3642a3

The reasons for the build failure are not
fully clear at the time of this writing;
apparently, a change in `pahole` is the core cause.
For details see the issue and in particular this comment:
https://github.com/NixOS/nixpkgs/issues/403985#issuecomment-2863338895
If that is correct and `pahole` gets fixed,
we can probably revert the commit at hand.

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