Handle all modules being serialized during LTO #3395

Job	Run time
spell_check	8s
duplicates	3s
build_system	24s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	6m 29s
build (gcc-15.deb, --extended-regex-example-tests)	6m 10s
build (gcc-15.deb, --test-libcore)	14m 3s
build (gcc-15-without-int128.deb, --extended-rand-tests)	8m 37s
build (gcc-15-without-int128.deb, --test-libcore)	13m 17s
build (gcc-15.deb, --projects)	13m 16s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	13m 51s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	14m 0s
build (gcc-15.deb, --extended-regex-tests)	11m 49s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	14m 46s
build (gcc-15.deb, --extended-rand-tests)	8m 15s
build (gcc-15.deb, --std-tests)	6m 3s
build (gcc-15-without-int128.deb, --extended-regex-tests)	12m 17s
build (gcc-15-without-int128.deb, --projects)	12m 42s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	14m 41s
build (gcc-15-without-int128.deb, --std-tests)	6m 18s
success	3s
	2h 57m 12s

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