Change the implementation of supports_parallel to signal tha cg_gcc is not thread safe. #1575

Job	Run time
build (--std-tests)	2m 6s
build (--mini-tests)	49s
build (--test-libcore)	1m 29s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	50s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	1m 30s
success_gcc12	2s
	6m 46s

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