Move libgccjit.so in the sysroot #3101

Job	Run time
spell_check	10s
duplicates	3s
build_system	24s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	13m 32s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	13m 46s
build (gcc-15.deb, --std-tests)	6m 0s
build (gcc-15.deb, --extended-regex-example-tests)	6m 24s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	6m 22s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	13m 8s
build (gcc-15-without-int128.deb, --extended-regex-tests)	11m 32s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	14m 27s
build (gcc-15-without-int128.deb, --std-tests)	5m 52s
build (gcc-15-without-int128.deb, --test-libcore)	16m 44s
build (gcc-15.deb, --extended-regex-tests)	11m 54s
build (gcc-15-without-int128.deb, --projects)	11m 12s
build (gcc-15-without-int128.deb, --extended-rand-tests)	8m 43s
build (gcc-15.deb, --projects)	10m 58s
build (gcc-15.deb, --extended-rand-tests)	8m 37s
build (gcc-15.deb, --test-libcore)	16m 42s
success	4s
	2h 56m 34s

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