GNUmakefile: Add a value for cross-build

[oech3]

uudoc needs different build RUSTFLAGS for cross-build.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[sylvestre]

Could you please add a test in github action to make sure we don't regress in the future?

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[oech3]

@Ecordonnier I don't know pkg names of Ubuntu well. Needs your help.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[sylvestre]

you are mixing changes {} => () and the rest of the change
could you please rewrite the stack to have commits doing one change

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[Ecordonnier]

Can't you just remove CARGOFLAGS from the call which is building uudocs instead of adding a new variable for the target? I would expect that those CARGOFLAGS would be for the target build, not for tools which are compiled for the host architecture.

[oech3]

Can't you just remove CARGOFLAGS from the call which is building uudocs instead of adding a new variable for the target?

No. Because it would block reusing prebuilt crate caches from coreutils (at non-cross build).

[oech3]

Because it would block reusing prebuilt crate caches from coreutils

@aaron-ang Is uudoc changed by feat_selinux or feat_systemd_logind and removing them cause double build?