gcc: do not allow version skew when cross-building gcc

When `gcc` is cross-built (`build` != `target` && `host` == `target`)
`gcc` assumes that it has a compatible cross-compiler in the environment
that can build target libraries. Version of a cross-compiler has to
match the compiler being cross-built as libraries frequently use fresh
compiler features, like `-std=c++26` or target-specific types like
`_Bfloat16`.

Version mismatch causes build failures like:

    https://github.com/NixOS/nixpkgs/issues/351905

Similar problems (but on a smaller scale) happen when a `gcc`
cross-compiler is built (`build` == `host` && `host` != `target`) built
by a mismatching version of a native compiler (`build` == `host` &&
`host` == `target`). That was worked around by forcing `gcc9Stdenv` for
older compiler versions.

Let's fix both problems by requiring the same compiler version for
cross-case.

Closes: https://github.com/NixOS/nixpkgs/issues/351905

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