When adding a mask or accept keywords entry for some version of a
toolchain package (gcc, libc, gdb, binutils or kernel headers), it
can't be done by just doing it, for example, for sys-devel/gcc. Both
cross-{x86_64,aarch64}-cros-linux-gnu/gcc needs to be
masked/keyworded, otherwise crossdev will pick up the latest stable
version for cross-{x86_64,aarch64}-cros-linux-gnu/gcc and this choice
is not affected by masks or accept keywords of sys-devel/gcc.
This situation does not happen all that often, but when it happens,
it's usually hard to remember to handle also the cross toolchain
packages. Forgetting to do so leads to weird issues. So instead of
telling crossdev to use the latest stable versions of cross toolchain
packages, we will tell it to use specific versions that match the
version of plain packages.
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