WIP: Add flag to use the LLVM sysroot #2101

Job	Run time
build (--std-tests)	51s
build (--mini-tests)	51s
build (--test-libcore)	50s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	58s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	1m 36s
success_gcc12	4s
	5m 10s

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